Is there a point where you're optimally filling your gas tank?

[oiprocs]

As I was filling up yesterday, I wondered if filling the tank completely was a good idea. I figured the weight of a full tank might be slowing me down, thus making the engine work harder, thus less gas mileage, etc.

Of course, you can't fill it too low, because that's not economical. You'll be visiting the gas station way too often.

Maybe filling your tank halfway is the optimum level? Sort of like how ~55-60 mph gives you the best gas mileage?

Edit:

I drive an '02 Civic, and I reside in San Diego ... if that makes the question any easier.

 

[JulesMaximus]

In a 3,500lb vehicle 20-30lbs of gas will have a negligible impact on fuel economy.

Edit-In the end you'll just be spending more time at the gas station breathing gasoline fumes.

 

[LTC8K6]

~6 pounds per gallon, so even a full 20 gallon tank is only 120 pounds or so.

I suppose losing 60 pounds might be of some small benefit, but probably not enough to justify/offset having to make extra trips to the pump.

 

[mwmorph]

If I remember correctly, the rule of thumb was every extra 200lbs=2% mpg decrease. The most fuel efficient, assuming you work at a gas station, would to fill just exactly enough to get to work and home, and of course park right next to the pump so you don't have to re-start your car after fueling.

Realistically, just keep filling to full.

 

[fleabag]

Originally posted by: JulesMaximus
In a 3,500lb vehicle 20-30lbs of gas will have a negligible impact on fuel economy.

Edit-In the end you'll just be spending more time at the gas station breathing gasoline fumes.

it's not 20-30lbs, more like 100lbs..

To the op: If you want to improve anything, try to improve the amount of gas you get per dollar and you can do this by filling up at night or very early morning where the gas is its coldest. If you've got a 25 gallon tank, filling only half of it could mean a mileage improvement but if you've got like a 10 gallon tank, it's not likely to result in any measurable improvement in mileage as 5 gallons of gas isn't THAT heavy compared to like 15 or 20 gallons of gas. Vehicles I drive have like 18-22.5 gallon tanks and I generally fill them half way to save weight. Also it depends, how often do you fill up anyhow? If you don't fill up that often, then filling up your tank to only half way is perfectly reasonable but if you drive a lot of miles on the highway and go through a tank pretty often, then filling up all the way would make more sense.

Weight really affects CITY mileage more than highway mileage, so if you're going to do a lot of highway driving, the no brainer is to fill up all the way even if it's a large tank. Otherwise there could be a measurable benefit to having a partially filled tank if you're doing a lot of stop and go especially if you're able to shed 100lbs worth of weight.

 

[DesiPower]

reminds me of formula1 racing. pit strategy wins the race for them. 2 stops or 3 stops. 2 stops will save 1 stop by heavier car: 3 stops lighter car but 1 extra stop... in both the cases you got the most advantage in the last 10 or so laps. 3 stop also gave you the advantage of using softer compound tires!
but eventually most of the teams choose 2 stop strategy. 3 stop never succeeded...

 

[JulesMaximus]

Originally posted by: fleabag

Originally posted by: JulesMaximus
In a 3,500lb vehicle 20-30lbs of gas will have a negligible impact on fuel economy.

Edit-In the end you'll just be spending more time at the gas station breathing gasoline fumes.

it's not 20-30lbs, more like 100lbs..

To the op: If you want to improve anything, try to improve the amount of gas you get per dollar and you can do this by filling up at night or very early morning where the gas is its coldest. If you've got a 25 gallon tank, filling only half of it could mean a mileage improvement but if you've got like a 10 gallon tank, it's not likely to result in any measurable improvement in mileage as 5 gallons of gas isn't THAT heavy compared to like 15 or 20 gallons of gas. Vehicles I drive have like 18-22.5 gallon tanks and I generally fill them half way to save weight. Also it depends, how often do you fill up anyhow? If you don't fill up that often, then filling up your tank to only half way is perfectly reasonable but if you drive a lot of miles on the highway and go through a tank pretty often, then filling up all the way would make more sense.

Weight really affects CITY mileage more than highway mileage, so if you're going to do a lot of highway driving, the no brainer is to fill up all the way even if it's a large tank. Otherwise there could be a measurable benefit to having a partially filled tank if you're doing a lot of stop and go especially if you're able to shed 100lbs worth of weight.

My car holds 15 gallons of gasoline. Half of that would be 7.5 gallons or about 45lbs of fuel.

My point stands. It's not going to make much, if any, difference at all and you're wasting your time only filling halfway. Filling up all the way ALWAYS makes more sense.

 

[JulesMaximus]

Originally posted by: DesiPower
reminds me of formula1 racing. pit strategy wins the race for them. 2 stops or 3 stops. 2 stops will save 1 stop by heavier car: 3 stops lighter car but 1 extra stop... in both the cases you got the most advantage in the last 10 or so laps. 3 stop also gave you the advantage of using softer compound tires!
but eventually most of the teams choose 2 stop strategy. 3 stop never succeeded...

Those cars weigh about 1200lbs though and consume fuel at the rate of about 1 gallon per lap and the extra weight of 1 gallon of fuel can cost you 1 or 2 tenths of a second a lap. In F1, it's all about the weight and the lap times. More weight equals slower lap times...all else being equal of course.

 

[Zenmervolt]

Originally posted by: fleabag
To the op: If you want to improve anything, try to improve the amount of gas you get per dollar and you can do this by filling up at night or very early morning where the gas is its coldest.

Oh dear God in Heaven not this bullshit again. The storage tanks are underground far enough that there is no significant temperature fluctuation. The fact that this myth keeps getting passed around is a testament to the frightening lack of basic scientific knowledge among the general public.

Something I posted on another forum a long time ago that still holds true:

Ever gone into a root cellar in the middle of summer when it's ~100 out? It'll be ~60 degrees in the root cellar, and a root cellar isn't even completely burried. Put something 10-15 feet underground like a gas station's tanks are and you're talking about maybe, maybe a 15 degree swing in temperature from the coldest time in winter to the hottest time in summer. Daily fluctuation would be less than 1 degree.

For comparison, using measurements from a root cellar in Alberta, outside temperatures were less than -40 Fahrenheit and inside the root cellar the temperature was around 35 degrees (i.e. above freezing) but in the summer when temperatures were in the 90's the root cellar's temperature was around 50 degrees.

A 30 degree increase in temperature (from 60 degrees to 90 degrees) will cause an increase in volume of one gallon of gasoline of 4 cubic inches. (From 231 cubic inches to 235 cubic inches.) Half of that increase, which represents the temperature change at tank depth equivalent to an above-ground swing of a staggering 130 degrees (remember that in the above example the below-ground temp changed from 35 degrees to 50 degrees when the above-ground temp changed from -40 to +90), would mean a change in volume from 231 cubic inches to 233 cubic inches. That's a change of 0.8% (0.008gallons) in volume from the coldest day of winter to the hottest day of summer. Change during the average day will be at most a single degree of fluctuation. Not enough to make any measureable difference.

ZV

 

[PricklyPete]

I have to agree with Jules...don't think it makes much difference in reality...at least not enough to make it worthwhile.

That being said...I never put more than $20 in my car at a time unless I'm on a long trip. I drive so few miles that $20 of gas generally lasts me a month unless I traveling for work a lot that month which may make it last even longer. The reason I only put $20 in at a time is I feel like the gas will get "old". Probably a stupid reason...but I do it anyway.

 

[HardcoreRobot]

my time is worth more than any gas savings that would provide

 

[Zenmervolt]

Originally posted by: PricklyPete
The reason I only put $20 in at a time is I feel like the gas will get "old". Probably a stupid reason...but I do it anyway.

The major gasoline brands use additives that allow gasoline to remain stable for at least a year under "normal" conditions. "Normal" means a properly sealing fuel system that isn't leaking and venting to atmosphere.

ZV

 

[overst33r]

Originally posted by: mwmorph
If I remember correctly, the rule of thumb was every extra 200lbs=2% mpg decrease. The most fuel efficient, assuming you work at a gas station, would to fill just exactly enough to get to work and home, and of course park right next to the pump so you don't have to re-start your car after fueling.

Realistically, just keep filling to full.

I've read for every 10% weight lost, fuel economy improves by 7%.

 

[PlasmaBomb]

Originally posted by: DesiPower
reminds me of formula1 racing. pit strategy wins the race for them. 2 stops or 3 stops. 2 stops will save 1 stop by heavier car: 3 stops lighter car but 1 extra stop... in both the cases you got the most advantage in the last 10 or so laps. 3 stop also gave you the advantage of using softer compound tires!
but eventually most of the teams choose 2 stop strategy. 3 stop never succeeded...

Changes in the rules changed pit stop strategies.

 

[fleabag]

Originally posted by: Zenmervolt

Originally posted by: fleabag
To the op: If you want to improve anything, try to improve the amount of gas you get per dollar and you can do this by filling up at night or very early morning where the gas is its coldest.

Oh dear God in Heaven not this bullshit again. The storage tanks are underground far enough that there is no significant temperature fluctuation. The fact that this myth keeps getting passed around is a testament to the frightening lack of basic scientific knowledge among the general public.

Something I posted on another forum a long time ago that still holds true:

Ever gone into a root cellar in the middle of summer when it's ~100 out? It'll be ~60 degrees in the root cellar, and a root cellar isn't even completely burried. Put something 10-15 feet underground like a gas station's tanks are and you're talking about maybe, maybe a 15 degree swing in temperature from the coldest time in winter to the hottest time in summer. Daily fluctuation would be less than 1 degree.

For comparison, using measurements from a root cellar in Alberta, outside temperatures were less than -40 Fahrenheit and inside the root cellar the temperature was around 35 degrees (i.e. above freezing) but in the summer when temperatures were in the 90's the root cellar's temperature was around 50 degrees.

A 30 degree increase in temperature (from 60 degrees to 90 degrees) will cause an increase in volume of one gallon of gasoline of 4 cubic inches. (From 231 cubic inches to 235 cubic inches.) Half of that increase, which represents the temperature change at tank depth equivalent to an above-ground swing of a staggering 130 degrees (remember that in the above example the below-ground temp changed from 35 degrees to 50 degrees when the above-ground temp changed from -40 to +90), would mean a change in volume from 231 cubic inches to 233 cubic inches. That's a change of 0.8% (0.008gallons) in volume from the coldest day of winter to the hottest day of summer. Change during the average day will be at most a single degree of fluctuation. Not enough to make any measureable difference.

ZV

It's not a myth, it HAS been proven to be true, there is a reason why in canada they have temperature compensating fuel pumps but not here. Since gasoline is calculated based upon temperatures of 60F, when the temperature goes above that, you get less gasoline that you're paying for, and when the temperature goes below that, you get more gasoline than you paid for which is why they were so eager to install the temperature compensating pumps in canada but not here since it's hotter here and colder there. This is NOT A MYTH you're just uneducated and misinformed about this.

THIS IS NOT A MYTH.

 

[PricklyPete]

I'll have to substitute a lawn chair for this desk chair...this is going to get good.

 

[PhoKingGuy]

Originally posted by: fleabag

Originally posted by: Zenmervolt

Originally posted by: fleabag
To the op: If you want to improve anything, try to improve the amount of gas you get per dollar and you can do this by filling up at night or very early morning where the gas is its coldest.

Oh dear God in Heaven not this bullshit again. The storage tanks are underground far enough that there is no significant temperature fluctuation. The fact that this myth keeps getting passed around is a testament to the frightening lack of basic scientific knowledge among the general public.

Something I posted on another forum a long time ago that still holds true:

Ever gone into a root cellar in the middle of summer when it's ~100 out? It'll be ~60 degrees in the root cellar, and a root cellar isn't even completely burried. Put something 10-15 feet underground like a gas station's tanks are and you're talking about maybe, maybe a 15 degree swing in temperature from the coldest time in winter to the hottest time in summer. Daily fluctuation would be less than 1 degree.

For comparison, using measurements from a root cellar in Alberta, outside temperatures were less than -40 Fahrenheit and inside the root cellar the temperature was around 35 degrees (i.e. above freezing) but in the summer when temperatures were in the 90's the root cellar's temperature was around 50 degrees.

A 30 degree increase in temperature (from 60 degrees to 90 degrees) will cause an increase in volume of one gallon of gasoline of 4 cubic inches. (From 231 cubic inches to 235 cubic inches.) Half of that increase, which represents the temperature change at tank depth equivalent to an above-ground swing of a staggering 130 degrees (remember that in the above example the below-ground temp changed from 35 degrees to 50 degrees when the above-ground temp changed from -40 to +90), would mean a change in volume from 231 cubic inches to 233 cubic inches. That's a change of 0.8% (0.008gallons) in volume from the coldest day of winter to the hottest day of summer. Change during the average day will be at most a single degree of fluctuation. Not enough to make any measureable difference.

ZV

It's not a myth, it HAS been proven to be true, there is a reason why in canada they have temperature compensating fuel pumps but not here. Since gasoline is calculated based upon temperatures of 60F, when the temperature goes above that, you get less gasoline that you're paying for, and when the temperature goes below that, you get more gasoline than you paid for which is why they were so eager to install the temperature compensating pumps in canada but not here since it's hotter here and colder there. This is NOT A MYTH you're just uneducated and misinformed about this.

THIS IS NOT A MYTH.

You know you have a full team of engineers at Hyundai USA that follow your BS regularly? You're getting a wall of fame apparently.

 

[ChaosDivine]

Just fill it full up. The only time I deviate from that is when spending prepaid cards from rebates - usually fill ~$20-$25 each time.
By far the easiest way to spend every last cent on those damned things, eg. I can ask for $12.34 of gas and get exactly that. No over pre-authorization BS.

 

[caspur]

Post for fleabag:

Go to any gas station. Look at the Veeder-root printouts for temperature. The variance is minimal and is not affected by air temperature. Now I have 10,000 gallon tanks, so perhaps with smaller tanks you will get a larger variance, but it won't have any appreciable difference in the amount you can get on fill up.

Actually, my deliveries (which are from tanker trucks) would be affected far more by the outside temperature than the fuel that is already in the underground tanks.

 

[zerocool84]

Originally posted by: fleabag

Originally posted by: Zenmervolt

Originally posted by: fleabag
To the op: If you want to improve anything, try to improve the amount of gas you get per dollar and you can do this by filling up at night or very early morning where the gas is its coldest.

Oh dear God in Heaven not this bullshit again. The storage tanks are underground far enough that there is no significant temperature fluctuation. The fact that this myth keeps getting passed around is a testament to the frightening lack of basic scientific knowledge among the general public.

Something I posted on another forum a long time ago that still holds true:

Ever gone into a root cellar in the middle of summer when it's ~100 out? It'll be ~60 degrees in the root cellar, and a root cellar isn't even completely burried. Put something 10-15 feet underground like a gas station's tanks are and you're talking about maybe, maybe a 15 degree swing in temperature from the coldest time in winter to the hottest time in summer. Daily fluctuation would be less than 1 degree.

For comparison, using measurements from a root cellar in Alberta, outside temperatures were less than -40 Fahrenheit and inside the root cellar the temperature was around 35 degrees (i.e. above freezing) but in the summer when temperatures were in the 90's the root cellar's temperature was around 50 degrees.

A 30 degree increase in temperature (from 60 degrees to 90 degrees) will cause an increase in volume of one gallon of gasoline of 4 cubic inches. (From 231 cubic inches to 235 cubic inches.) Half of that increase, which represents the temperature change at tank depth equivalent to an above-ground swing of a staggering 130 degrees (remember that in the above example the below-ground temp changed from 35 degrees to 50 degrees when the above-ground temp changed from -40 to +90), would mean a change in volume from 231 cubic inches to 233 cubic inches. That's a change of 0.8% (0.008gallons) in volume from the coldest day of winter to the hottest day of summer. Change during the average day will be at most a single degree of fluctuation. Not enough to make any measureable difference.

ZV

It's not a myth, it HAS been proven to be true, there is a reason why in canada they have temperature compensating fuel pumps but not here. Since gasoline is calculated based upon temperatures of 60F, when the temperature goes above that, you get less gasoline that you're paying for, and when the temperature goes below that, you get more gasoline than you paid for which is why they were so eager to install the temperature compensating pumps in canada but not here since it's hotter here and colder there. This is NOT A MYTH you're just uneducated and misinformed about this.

THIS IS NOT A MYTH.

You're the last person to give any type of insight on anything relating to cars.

 

[fleabag]

Originally posted by: PhoKingGuy

Originally posted by: fleabag

Originally posted by: Zenmervolt

Originally posted by: fleabag
To the op: If you want to improve anything, try to improve the amount of gas you get per dollar and you can do this by filling up at night or very early morning where the gas is its coldest.

Oh dear God in Heaven not this bullshit again. The storage tanks are underground far enough that there is no significant temperature fluctuation. The fact that this myth keeps getting passed around is a testament to the frightening lack of basic scientific knowledge among the general public.

Something I posted on another forum a long time ago that still holds true:

Ever gone into a root cellar in the middle of summer when it's ~100 out? It'll be ~60 degrees in the root cellar, and a root cellar isn't even completely burried. Put something 10-15 feet underground like a gas station's tanks are and you're talking about maybe, maybe a 15 degree swing in temperature from the coldest time in winter to the hottest time in summer. Daily fluctuation would be less than 1 degree.

For comparison, using measurements from a root cellar in Alberta, outside temperatures were less than -40 Fahrenheit and inside the root cellar the temperature was around 35 degrees (i.e. above freezing) but in the summer when temperatures were in the 90's the root cellar's temperature was around 50 degrees.

A 30 degree increase in temperature (from 60 degrees to 90 degrees) will cause an increase in volume of one gallon of gasoline of 4 cubic inches. (From 231 cubic inches to 235 cubic inches.) Half of that increase, which represents the temperature change at tank depth equivalent to an above-ground swing of a staggering 130 degrees (remember that in the above example the below-ground temp changed from 35 degrees to 50 degrees when the above-ground temp changed from -40 to +90), would mean a change in volume from 231 cubic inches to 233 cubic inches. That's a change of 0.8% (0.008gallons) in volume from the coldest day of winter to the hottest day of summer. Change during the average day will be at most a single degree of fluctuation. Not enough to make any measureable difference.

ZV

It's not a myth, it HAS been proven to be true, there is a reason why in canada they have temperature compensating fuel pumps but not here. Since gasoline is calculated based upon temperatures of 60F, when the temperature goes above that, you get less gasoline that you're paying for, and when the temperature goes below that, you get more gasoline than you paid for which is why they were so eager to install the temperature compensating pumps in canada but not here since it's hotter here and colder there. This is NOT A MYTH you're just uneducated and misinformed about this.

THIS IS NOT A MYTH.

You know you have a full team of engineers at Hyundai USA that follow your BS regularly? You're getting a wall of fame apparently.

I don't give a shit about your friends in automotive engineering school waiting to get a job a Hyundai.

Originally posted by: caspur
Post for fleabag:

Go to any gas station. Look at the Veeder-root printouts for temperature. The variance is minimal and is not affected by air temperature. Now I have 10,000 gallon tanks, so perhaps with smaller tanks you will get a larger variance, but it won't have any appreciable difference in the amount you can get on fill up.

Actually, my deliveries (which are from tanker trucks) would be affected far more by the outside temperature than the fuel that is already in the underground tanks.

Well, yes and no. I'm not saying it's a huge deal, in fact it's probably a matter of a few cents per fill up, and to a lot of people that's "not a big deal" but when you multiply that by each fill up, it can add up and it actually has resulted in millions of consumers being "ripped off". However since gas stations use that extra profit to function and set their gas prices accordingly, it's actually not as egregious as it would seem. So really it's one of those things where if you want to take advantage of it, go ahead but not doing so isn't going to be THAT big of a deal. I never said it was a big deal despite me making it seem like I am, I'm just saying that to say that there isn't a measurable difference between gasoline at 60F and 80F+ is a lie and can theoretically add up over time.

If you care enough to take advantage of filling up in the morning, be my guest and go ahead, you will save money, however it's really up to you to choose between inconvenience of doing this vs. filling up when convenient. Also if you're going to drive when you normally wouldn't be driving JUST to fill up and not accomplish any other errand that you would already be doing anyways, then it's very possible you'll negate any savings you could have had.

Another thing to consider is that your car will burn more fuel when it's cold outside than when it's warm so if you have a vehicle that particularly benefits from hot weather, then going for a drive in the 60 or 50F morning just to save on fuel vs. filling up during the 90 or 100F day could easily result in net LOSS for you. The most "optimal" thing you could possibly do would be instead of filling up your car on your way home from work, just drive home and then the next morning on your way to work, fill up your gas tank.

 

[Cuda1447]

Oh geez, this thread is going places. Not to hate on everyones debate, but don't you think how you drive would have a much larger effect than any of this non-sense? I wouldn't even consider ANY of these methods mentioned even if they were true, unless I drove like a granny and was pinching every penny I had. One time gunning it at the light or to get onto a highway and everything you saved by filling up your tank 3/8ths of the way at 1 am on the coldest night of the week, is wasted.

 

[JulesMaximus]

Originally posted by: mariok2006

Originally posted by: mwmorph
If I remember correctly, the rule of thumb was every extra 200lbs=2% mpg decrease. The most fuel efficient, assuming you work at a gas station, would to fill just exactly enough to get to work and home, and of course park right next to the pump so you don't have to re-start your car after fueling.

Realistically, just keep filling to full.

I've read for every 10% weight lost, fuel economy improves by 7%.

10% of 3,500lbs is 350lbs. So, if that's true and you could shed 350lbs from a car that typically gets 20mpg city you might see an increase of 1-1.5mpg.

The heavier the vehicle, the worse it gets. Take a 5,000lb truck and you'd have to shed 500lbs of weight from it to save about 1mpg.

Either way, a half a tank of fuel isn't going to amount to any noticeable savings in fuel.

 

[fleabag]

Originally posted by: Cuda1447
Oh geez, this thread is going places. Not to hate on everyones debate, but don't you think how you drive would have a much larger effect than any of this non-sense? I wouldn't even consider ANY of these methods mentioned even if they were true, unless I drove like a granny and was pinching every penny I had. One time gunning it at the light or to get onto a highway and everything you saved by filling up your tank 3/8ths of the way at 1 am on the coldest night of the week, is wasted.

We're just answering the OP's question, for all we know the op is already working towards max mileage. Also nothing is wasted, I mean what if the OP filled up at 120F day with the gas at 100F and then gunned the engine at 1am? He'd lose in both situations, but if the guy saved in one area and wasted in another, at least he saved in one area. That's kinda like saying if I waste money doing this, why not waste money doing that? Money saved is money saved, why should one thing negate another? I mean sure you don't want to spend all of your time pinching pennies and then splurge on a Gucci bag but at the same time, gunning the engine that one time or three times during the tank of gas is not the same as driving your car like it's a race car ALL THE TIME.

 

[PhoKingGuy]

Originally posted by: fleabag

Originally posted by: Cuda1447
Oh geez, this thread is going places. Not to hate on everyones debate, but don't you think how you drive would have a much larger effect than any of this non-sense? I wouldn't even consider ANY of these methods mentioned even if they were true, unless I drove like a granny and was pinching every penny I had. One time gunning it at the light or to get onto a highway and everything you saved by filling up your tank 3/8ths of the way at 1 am on the coldest night of the week, is wasted.

We're just answering the OP's question, for all we know the op is already working towards max mileage. Also nothing is wasted, I mean what if the OP filled up at 120F day with the gas at 100F and then gunned the engine at 1am? He'd lose in both situations, but if the guy saved in one area and wasted in another, at least he saved in one area. That's kinda like saying if I waste money doing this, why not waste money doing that? Money saved is money saved, why should one thing negate another? I mean sure you don't want to spend all of your time pinching pennies and then splurge on a Gucci bag but at the same time, gunning the engine that one time or three times during the tank of gas is not the same as driving your car like it's a race car ALL THE TIME.

This past weekend, I drove my GTI (yes I got it back) from Orange County to Berkeley. I filled up in OC before I left (about 70 degrees at 4:45 AM) and got 415 miles to that tank. I left from Sacramento to drive back down at 3PM and filled before I left (103 degrees), I got.....413 miles to that tank.

The difference of two miles could be me pressing my foot down a bit harder to pass a truck than anything, and i'll admit i was going faster on the way back than the way up.