Driving speed and saving money

palswim

Golden Member
Nov 23, 2003
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www.palswim.net
This thread and this site inspired (or prompted) this thread.

People are always telling you ways to save money on gas and one way I hear often is to slow down. This, of course meaning on the freeway/highway. But, what most people don't think enough about is that by slowing down, you also lose time, which is of (greater, I believe) value than money.




Here's an example which will lead to a not-too-difficult formula:
Put a value on your time. Let's say $15/hr is what you determine. (Some of you may think this is too low, a few may think it's too high - adjust accordingly then.)
And, again a gross simplification, our gas costs $3 per gallon.

Say driving at 60 mph, your car's fuel efficiency is 25 mpg.
Then, a 30 mile trip would take half an hour (30 mi/60 mph) and would use 1.2 gallons of gas (30 mi/25 mpg).
So, $15/hr*(.5 hr) + 1.2 gal*$3/gal = $7.5 + $3.6 = $11.1
The trip costs you $11.10.

And now, let's be conservative and put your car's performance at 22 mpg at 70 mph (these numbers won't be as pretty).
A 30 mile trip would take 26 minutes (.43 hr = 30 mi/70 mph) and consume 1.4 gallons of gas (30 mi/22 mpg).
$15/hr*(.43 hr) + 1.4 gal*$3/gal = $6.43 + $4.2 = $10.6
The trip costs you $10.60.


Yes, this calculation requires some estimation/fudging, but I'm pretty sure those mpg numbers are a lot closer than I've put them, but perhaps not. And realize that these numbers worked in a speeders' favor for this example; if you had a gas-guzzling SUV or lower value on your time or gas prices were higher, then driving slower might really save you money. This is just something to think about.



As far as a formula goes, it's not too complicated:
h = the value you put on your time
g = fuel cost (per unit)
d = distance covered (this is a constant, so it's not even necessary in the calculation for comparison)
r1 = rate/speed/velocity for calculation #1
r2 = rate/speed/velocity for calculation #2
f1 = fuel efficiency for calculation #1
f2 = fuel efficiency for calculation #2

Calculation #1 Cost = h*d/r1 + g*d/f1
Calculation #2 Cost = h*d/r2 + g*d/f2

Remember, the objective is to minimize costs, or "save you money" (it's the reason driving slower sounded good in the first place, right?).

I know somehow, speed and fuel efficiency are related (above a certain speed, fuel efficiency starts decreasing, faster and faster, too), but I don't know the relation, so you just have to estimate.




In conclusion, this thread is not an encouragement for people to start driving like maniacs (I'm not even advocating anyone driving faster at all), but just a counterbalance to all the people who love to tell you that driving slower saves you money.
 

jagec

Lifer
Apr 30, 2004
24,442
6
81
If slowing down really saved gas, how come city mileage figures are so much lower than highway?:p

Real fuel-saving advice: Be very light on the gas, and even lighter on the brake. Don't drop below 30MPH, or go over 70.

Of course, that's completely impractical, but it WILL save fuel.

Interesting calculations, though...I'll have to bring them up the next time I get pulled over for speeding.
 

Ricemarine

Lifer
Sep 10, 2004
10,507
0
0
Wasn't the reason why most freeways have the speed limit at 55 mph was because that's where you get the most miles/gallon?... Some have 60 mph just for those people who commute.

So wouldn't you get less miles/gallon if you went faster than the 60 mph?...
 

Kelemvor

Lifer
May 23, 2002
16,928
8
81
The main reason mileage goes down once you get upwards of 45-55mph is because of wind resistance and the gearing of your car. Most cars are in their top gear around 45 mph. THat means that as you go faster from that speed, you are using more gas because there's no next gear to shift into and because the engine has to work harder to push the car through the air faster.

If you have a car with a digital display, find a stretch of road that's not too hilly and set your cruise control at 55 and see what mileage you get. Then set your cruise at 65 or 70 and drive the same route. Your mileage will be lower. But it's not all that much lower so most people just don't worry about it.
 

PingSpike

Lifer
Feb 25, 2004
21,751
595
126
Aren't those 55 mph figures based on cars from the 70s? I thought modern cars were geared differently.

Oh, and I ain't driving 55 on freeways that are 65 around here. I'm going to be 'that guy'.
 

dxkj

Lifer
Feb 17, 2001
11,772
2
81
At 65 mph I can get 30 mpg, at 75 mpg I get 25mpg

My commute is 85 miles each way.


So 170 miles

1.0833 miles per minute
1.25 miles per minute

157 minutes ~ 2 hours 37 minutes ~ 5.67 gallons
136 minutes ~ 2 hours 16 minutes ~ 6.8 gallons


21 minutes saved at the cost of 1.13 gallons

We pay like $3.15 a gallon


$3.60 for 21 minutes, that is 17 cents per minute, so $10.17 an hour cost of speeding
and a risk of increased getting pulled over :)









 

tlam617

Senior member
Jun 24, 2003
369
0
0
Originally posted by: jagec
If slowing down really saved gas, how come city mileage figures are so much lower than highway?:p

Real fuel-saving advice: Be very light on the gas, and even lighter on the brake. Don't drop below 30MPH, or go over 70.

Of course, that's completely impractical, but it WILL save fuel.

Interesting calculations, though...I'll have to bring them up the next time I get pulled over for speeding.


I agree. You can go to whichever speed so long as you don't brake that much. If you're a good driver, you won't need to worry about a speeding ticket even if you are speeding.
 

PingSpike

Lifer
Feb 25, 2004
21,751
595
126
Originally posted by: tlam617
Originally posted by: jagec
If slowing down really saved gas, how come city mileage figures are so much lower than highway?:p

Real fuel-saving advice: Be very light on the gas, and even lighter on the brake. Don't drop below 30MPH, or go over 70.

Of course, that's completely impractical, but it WILL save fuel.

Interesting calculations, though...I'll have to bring them up the next time I get pulled over for speeding.


I agree. You can go to whichever speed so long as you don't brake that much. If you're a good driver, you won't need to worry about a speeding ticket even if you are speeding.

Oh, they don't care if you're a good driver.
 

BurnItDwn

Lifer
Oct 10, 1999
26,236
1,722
126
Originally posted by: palswim
This thread and this site inspired (or prompted) this thread.

People are always telling you ways to save money on gas and one way I hear often is to slow down. This, of course meaning on the freeway/highway. But, what most people don't think enough about is that by slowing down, you also lose time, which is of (greater, I believe) value than money.




Here's an example which will lead to a not-too-difficult formula:
Put a value on your time. Let's say $15/hr is what you determine. (Some of you may think this is too low, a few may think it's too high - adjust accordingly then.)
And, again a gross simplification, our gas costs $3 per gallon.

Say driving at 60 mph, your car's fuel efficiency is 25 mpg.
Then, a 30 mile trip would take half an hour (30 mi/60 mph) and would use 1.2 gallons of gas (30 mi/25 mpg).
So, $15/hr*(.5 hr) + 1.2 gal*$3/gal = $7.5 + $3.6 = $11.1
The trip costs you $11.10.

And now, let's be conservative and put your car's performance at 22 mpg at 70 mph (these numbers won't be as pretty).
A 30 mile trip would take 26 minutes (.43 hr = 30 mi/70 mph) and consume 1.4 gallons of gas (30 mi/22 mpg).
$15/hr*(.43 hr) + 1.4 gal*$3/gal = $6.43 + $4.2 = $10.6
The trip costs you $10.60.


Yes, this calculation requires some estimation/fudging, but I'm pretty sure those mpg numbers are a lot closer than I've put them, but perhaps not. And realize that these numbers worked in a speeders' favor for this example; if you had a gas-guzzling SUV or lower value on your time or gas prices were higher, then driving slower might really save you money. This is just something to think about.



As far as a formula goes, it's not too complicated:
h = the value you put on your time
g = fuel cost (per unit)
d = distance covered (this is a constant, so it's not even necessary in the calculation for comparison)
r1 = rate/speed/velocity for calculation #1
r2 = rate/speed/velocity for calculation #2
f1 = fuel efficiency for calculation #1
f2 = fuel efficiency for calculation #2

Calculation #1 Cost = h*d/r1 + g*d/f1
Calculation #2 Cost = h*d/r2 + g*d/f2

Remember, the objective is to minimize costs, or "save you money" (it's the reason driving slower sounded good in the first place, right?).

I know somehow, speed and fuel efficiency are related (above a certain speed, fuel efficiency starts decreasing, faster and faster, too), but I don't know the relation, so you just have to estimate.




In conclusion, this thread is not an encouragement for people to start driving like maniacs (I'm not even advocating anyone driving faster at all), but just a counterbalance to all the people who love to tell you that driving slower saves you money.



Rather then being selfish and only thinking of yourself. Instead think of the big picture.
If EVERYBODY wasn't so lead footed and gained 13.6% better milage, then that's 13.6% less oil being used for auto fuel in the US. That's a good chunk. This means less stress on the production of oil worldwide. This means cheaper fuel prices, or at least a slowdown in the price increases. Also it means there will be less emissions as well, but I'm thinking mostly on a macroeconomic scale rather than an environmental one.

(I'm using 13.6% because 25 is ~13.6% more than 22.)
 

palswim

Golden Member
Nov 23, 2003
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www.palswim.net
Originally posted by: dafatha00
What about taking into account the possibility of getting a speeding ticket?

What's the possibility of getting a speeding ticket?

Depends where you are, I guess. Here in San Diego, the officers won't bat an eye if you're going under 75 or 80 (on the 65 mph part of the freeway).
 

palswim

Golden Member
Nov 23, 2003
1,049
0
71
www.palswim.net
Originally posted by: BurnItDwn
Rather then being selfish and only thinking of yourself. Instead think of the big picture.
If EVERYBODY wasn't so lead footed and gained 13.6% better milage, then that's 13.6% less oil being used for auto fuel in the US. That's a good chunk. This means less stress on the production of oil worldwide. This means cheaper fuel prices, or at least a slowdown in the price increases. Also it means there will be less emissions as well, but I'm thinking mostly on a macroeconomic scale rather than an environmental one.

(I'm using 13.6% because 25 is ~13.6% more than 22.)

If you want to not be selfish, don't drive a car. I wasn't advocating everyone speed up, but I was refuting the appeal to "Drive slower, it saves You money!", which is a selfish pitch to begin with.
 

palswim

Golden Member
Nov 23, 2003
1,049
0
71
www.palswim.net
Originally posted by: Ricemarine
Originally posted by: dafatha00
What about taking into account the possibility of getting a speeding ticket?

Right here?. :laugh:

From the article:
Don't be an aggressive driver. The freeway is not a NASCAR race.
I bet I know who they targeted this for!

also:
At 75 mph, Bob would use 2.5 gallons of gasoline. At $2.80/gallon, that would be $7.00 to go the same distance.

But it cost Bob $155.60 and he got there AFTER Bill! WHY?

Bob was ticketed for speeding!

Bob needs to move to California.
 

Eos

Diamond Member
Jun 14, 2000
3,463
17
81
I get an overall average of 22.9 mpg with my 3.0L V-6, 4 speed auto Toyota.

City only is closer to 20 mpg.

On a recent trip 2200+ mile trip, I drove an average of about 85 mph (75 mph zone) all interstate for about 250 miles. The car got 29.1 mpg on that tank.

385.6 - 14.0 - $41.05 - 27.54 - 125813 - $2.91
250.4 - 8.6 - $25.00 - 29.12 - 126064 - $2.89
406.2 - 14.5 - $44.01 - 28.01 - 126470 - $3.02
264.0 - 9.3 - $28.00 - 28.39 - 126734 - $2.97
420.5 - 16.2 - $44.50 - 25.96 - 127154 - $2.73
439.8 - 15.1 - $45.30 - 29.13 - 127594 - $2.98
369.7 - 15.4 - $47.25 - 24.01 - 127964 - $3.05

Miles on tank, gallons, cost, mpg, mileage on odometer, cost per gallon (regular unleaded)

I'm weary of hearing about high speeds resulting in lower mpg. It's not the case for me.
 

DrPizza

Administrator Elite Member Goat Whisperer
Mar 5, 2001
49,601
166
111
www.slatebrookfarm.com
I agree with the OP about personal savings, to a degree. But, a lot of the time, people driving faster really don't save a significant amount of time. They just tend to be nearer to the front of the line at the first red light after the off-ramp.
 

jpeyton

Moderator in SFF, Notebooks, Pre-Built/Barebones
Moderator
Aug 23, 2003
25,375
142
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I get great mileage with my 4 cylinder Accord, even at 85MPH.

If you're going to break the speed limit, go big or go home.
 

Kalvin00

Lifer
Jan 11, 2003
12,705
4
81
When I was driving to Nevada towing a 4-wheeler, when doing 55-60mph with AC on, I would get 21 mpg (this is a 2002 Dodge Ram 2500 Diesel truck). Going 72 or so, I would get 16-17.

That's my scientific data.
 

Eos

Diamond Member
Jun 14, 2000
3,463
17
81
Originally posted by: DrPizza
I agree with the OP about personal savings, to a degree. But, a lot of the time, people driving faster really don't save a significant amount of time. They just tend to be nearer to the front of the line at the first red light after the off-ramp.

Everyone doesn't live in a crowded metropolis. There are no interstates reaching my town. The closest is about an hour away. Only state 2 lane highways in my hood.

I save some time by driving faster when I do that, but I'm not dense about it. I don't think I'll arrive at my destination HOURS sooner. But saving time is more about not stopping. Ever. Piss in a bottle if you have to. ;)
 

Eos

Diamond Member
Jun 14, 2000
3,463
17
81
Originally posted by: Kalvin00
When I was driving to Nevada towing a 4-wheeler, when doing 55-60mph with AC on, I would get 21 mpg (this is a 2002 Dodge Ram 2500 Diesel truck). Going 72 or so, I would get 16-17.

That's my scientific data.

Were you running at the same mph between fill ups?