tips for better gas mileage with my manual transmission car?

[kitatech]

Interesting....I'll try 2-2.5rpm shifts for the next tank...

 

[ElFenix]

1) car not really known for fuel mileage to begin with
2) hills, especially if you have to stop on the hill or just after
3) CEL
4) not particularly long commute, and most it city (see #1)

 

[desura]

1) car not really known for fuel mileage to begin with
2) hills, especially if you have to stop on the hill or just after
3) CEL
4) not particularly long commute, and most it city (see #1)

I mean, a friend of mine who has an older model Jeep Cherokee says he gets 8mpg in the area.

I wonder how a Prius would fare on my terrain. I have heard of people getting like 35mpg. Or maybe the systems on it would make it well-suited for hills.

 

[Yuriman]

I mean, a friend of mine who has an older model Jeep Cherokee says he gets 8mpg in the area.

I wonder how a Prius would fare on my terrain. I have heard of people getting like 35mpg. Or maybe the systems on it would make it well-suited for hills.

35 in an Aveo, or 35 in a Prius?

 

[desura]

35 in an Aveo, or 35 in a Prius?

35 in the prius.

 

[Yuriman]

35 in the prius.

I'd be surprised if you could get as low as 35 in a Prius, tbh, even if you're playing stoplight hero. They're rated for like 50+ city.

 

[exdeath]

RPMs don't tell the whole story.

You need a vacuum gauge as well to monitor engine load if you are going to be an extreme hypermiler. Being at 1500 RPM at 0 vacuum for 3 days to climb a hill isn't going to do you any MPG favors.

3000 RPM at 15" vacuum is using less fuel than 1500 RPM at 0".

Fueling is determined by the intersection of load vs RPM and properly tuned engines will run fatter (more fuel) and with retarded ignition (less efficient) under higher load in lower RPM and lean out as RPM increases and load decreases. Low RPM high load is knock prone and all EFI systems will respond with more fueling and less timing to protect the engine in that extreme.

Low RPM and high load (eg being in too high a gear) places more stress on the engine parts and causes higher combustion temperatures and prolonged operation at those elevated conditions and requires a richer mixture to keep it cool and safe and prevent heat soak and pre-detonation. The farther away current engine torque is from demanded/required torque, the harder the engine is working to accelerate to equilibrium (eg: cruise state), and the more fuel it's consuming until it gets there in too high a gear.

Not being in the proper gear or RPM range is not going to change the required torque to conquer the road condition, it's just going to make the engine work harder and achieve the opposite of what is intended (maximizing MPG).

The injectors may only fire half as many times per second, but you are spending twice the time or longer accelerating AND doing so with a greater engine load and a richer mixture and less timing advance and greater engine abuse.

 

[desura]

RPMs don't tell the whole story.

You need a vacuum gauge as well to monitor engine load if you are going to be an extreme hypermiler. Being at 1500 RPM at 0 vacuum for 3 days to climb a hill isn't going to do you any MPG favors.

3000 RPM at 15" vacuum is using less fuel than 1500 RPM at 0".

Fueling is determined by the intersection of load vs RPM and properly tuned engines will run fatter (more fuel) and with retarded ignition (less efficient) under higher load in lower RPM and lean out as RPM increases and load decreases. Low RPM high load is knock prone and all EFI systems will respond with more fueling and less timing to protect the engine in that extreme.

Low RPM and high load (eg being in too high a gear) places more stress on the engine parts and causes higher combustion temperatures and prolonged operation at those elevated conditions and requires a richer mixture to keep it cool and safe and prevent heat soak and pre-detonation. The farther away current engine torque is from demanded/required torque, the harder the engine is working to accelerate to equilibrium (eg: cruise state), and the more fuel it's consuming until it gets there in too high a gear.

Not being in the proper gear or RPM range is not going to change the required torque to conquer the road condition, it's just going to make the engine work harder and achieve the opposite of what is intended (maximizing MPG).

The injectors may only fire half as many times per second, but you are spending twice the time or longer accelerating AND doing so with a greater engine load and a richer mixture and less timing advance and greater engine abuse.

Thank you for this. I have always wondered about it.

 

[Yuriman]

Thank you for this. I have always wondered about it.

Unfortunately, his interpretation is wrong. You ought to read the wiki article on Brake Specific Fuel Consumption.

BSFC, in a nutshell, is how much fuel is used for a given amount of power/work done.

Here is a BSFC map from a 1.9L Saturn DOHC engine:

Lower fuel consumption is better. Notice that after about 80% load, the efficiency begins to drop? That's where you're getting your fuel enrichment, generally. Below 80% load efficiency drops very sharply due to losses such as creating vacuum (THIS IS A BAD THING). Also note that the peak efficiency is between 2500 and 3000RPM. Most engines will have a peak efficiency around 80% load and between 2000 and 4000RPM, depending on their displacement and stroke to bore ratio.

 

[Yuriman]

Vacuum is a useful piece of the big picture, but it's nearly useless without context (if you don't know what your car's computer is doing with it). It's much better to measure the actual fuel being put in the cylinders than to look at vacuum.

EDIT: Here's a slightly more readable chart, that includes horsepower:

The red spot is the area of highest effiency (least fuel used per power created).

RPMs don't tell the whole story.

You need a vacuum gauge as well to monitor engine load if you are going to be an extreme hypermiler. Being at 1500 RPM at 0 vacuum for 3 days to climb a hill isn't going to do you any MPG favors.

Actually, it appears that at 1500RPM with 0" of vacuum, you're relatively close to peak effiency.


3000 RPM at 15" vacuum is using less fuel than 1500 RPM at 0".

Not true for this particular motor. Might be true in a smaller displacement engine (say, 600cc motorcycle engine) that doesn't produce torque until higher RPM.


Fueling is determined by the intersection of load vs RPM and properly tuned engines will run fatter (more fuel) and with retarded ignition (less efficient) under higher load in lower RPM and lean out as RPM increases and load decreases. Low RPM high load is knock prone and all EFI systems will respond with more fueling and less timing to protect the engine in that extreme.

Most engines do begin fuel enrichment as you approach zero vacuum, but modern engines have knock sensors and don't retard timing until they need to. A better way to think of "fueling" is how the car's computer actually does it - it reads the AFR with your O2 sensor, and adjusts the amount of fuel for your given amount of air entering the cylinder to reach its desired AFR. Adding fuel decreases cylinder temperature (it has a cooling effect) and increases power, because generally, stoichometric combustion does not produce the most power possible for a given amount of fuel. It's true that computers will pull timing and increase fuel at high load, but this doesn't generally offset the increased effiency you get from increased cylinder pressure, higher cylinder wall temperature (less heat lost through cylinder walls during combustion), less energy wasted creating vacuum, and less frictional losses from higher RPM to get your needed power.


Low RPM and high load (eg being in too high a gear) places more stress on the engine parts and causes higher combustion temperatures and prolonged operation at those elevated conditions and requires a richer mixture to keep it cool and safe and prevent heat soak and pre-detonation.

Why do you think engines have rev limiters? There's no "turnover point" where RPM begins to cause more stress, rather than less. Torque and RPM together put stress on engine components. Stresses on bearings and crank are highest at high RPM, high load. Reducing either reduces stress.


The farther away current engine torque is from demanded/required torque, the harder the engine is working to accelerate to equilibrium (eg: cruise state), and the more fuel it's consuming until it gets there in too high a gear.

Not sure what you're trying to say here.


Not being in the proper gear or RPM range is not going to change the required torque to conquer the road condition, it's just going to make the engine work harder and achieve the opposite of what is intended (maximizing MPG).

Horsepower is what is required to push your car along. This is why we have gears. Torque x RPM give horsepower. There's no such thing as "required torque". A 1L engine running at 5000RPM to make 100HP, geared to spin the tires at the same speed as a 2L engine running at 2500RPM to make 100HP, will make (effectively) the exact same power.


The injectors may only fire half as many times per second, but you are spending twice the time or longer accelerating AND doing so with a greater engine load and a richer mixture and less timing advance and greater engine abuse.

The injectors will fire half as often, and you'll need less than twice the time to get up to speed because of the increased efficiency at lower RPM/higher load, as is shown by the BSFC chart.

EDIT: There are some other factors, such as flamespeed (changes based on fuel used) and its relationship with stroke length vs RPM (piston speed) that affect efficiency, but since these are mechanically part of the engine's design, there's not a lot we, as drivers, can do with knowing about them that isn't better explained by looking at BSFC.

 

[kitatech]

Also note that the peak efficiency is between 2500 and 3000RPM. Most engines will have a peak efficiency around 80% load and between 2000 and 4000RPM, depending on their displacement and stroke to bore ratio.

....and that's where I run the Camry most of the time...sweet....

 

[Yuriman]

....and that's where I run the Camry most of the time...sweet....

Indeed.

Unfortunately, the engine in your Camry is "oversized" enough that you are at extremely low load if cruising at 2500-3000RPM, and in order to gear it to be at high load while cruising, you'd end up at something crazy like 900RPM (which is also moving away from peak BSFC for that particular design). The biggest way hybrids get better mileage is by decreasing displacement of their combustion engines so they cruise at a much higher percent load, and supplement acceleration with an electric motor. If one were to disable the hybrid functions of a Prius or Insight, gas mileage would be about the same (aside from regenerative braking, but that's still pretty small), it would just be a slug to drive because of its tiny gasoline engine.

You're shifting in the right spot though.

 

[kitatech]

MPG aside, the Camry's I4 engine revs so smoothly that staying between 2.5-3k often recalls the "spirit" of my old BMW 2002 that only had 4 MT gears.
...actually, the Toyo I4 is more like my old Plymouth Sapporo/Mitsu Galant with their counterbalanced shafts...THAT car's engine was smooth and fast too...55mph+ in 2nd before redlining was sweet...

...interestingly, the Bimmer got 22/32+mpg in the 70s and I didn't drive it any easier than I drive the Camry...but I don't baby any car...my cuz remarked back then in my Fiat Spider that I always liked to shift between 3 and 3.5k....my response was that THE CARS like to shift there...

 

[pauldun170]

Indeed.

Unfortunately, the engine in your Camry is "oversized" enough that you are at extremely low load if cruising at 2500-3000RPM, and in order to gear it to be at high load while cruising, you'd end up at something crazy like 900RPM (which is also moving away from peak BSFC for that particular design). The biggest way hybrids get better mileage is by decreasing displacement of their combustion engines so they cruise at a much higher percent load, and supplement acceleration with an electric motor. If one were to disable the hybrid functions of a Prius or Insight, gas mileage would be about the same (aside from regenerative braking, but that's still pretty small), it would just be a slug to drive because of its tiny gasoline engine.

You're shifting in the right spot though.

I shift even earlier than he does.
It helps that the base model Camry (2002-2006) only weighs 3000lbs.

 

[desura]

So update.

Filled up tonight. Got 29.3 MPG.

That's what, a 20% improvement?

I stopped engine braking is the biggest change I've made, since there is no decel fuel cutoff.

Increased tire pressure. Turns out the check engine light is about tire pressure. Most cars I've had were 32. These actually want 40. Most of my miles were on like 34 though.

Not too bad for 3/4 city, 1/4 highway.

 

[soccerballtux]

So update.

Filled up tonight. Got 29.3 MPG.

That's what, a 20% improvement?

I stopped engine braking is the biggest change I've made, since there is no decel fuel cutoff.

Increased tire pressure. Turns out the check engine light is about tire pressure. Most cars I've had were 32. These actually want 40. Most of my miles were on like 34 though.

Not too bad for 3/4 city, 1/4 highway.

no fuel trim on engine braking man that's messed but exactly what I would expect from Chevrolet.

I got fantastic Milagros 38mpg actually on hills. taking it up takes only a small % more of fuel but on the way down its no fuel.

on highway I found when i drive 70-72mph I get 32mpg vs 28-29 at 80mph. On a big trip that's worth the savings

I overinflate tires too. worth it for savings and handling.

 

[exar333]

no fuel trim on engine braking man that's messed but exactly what I would expect from Chevrolet.

I got fantastic Milagros 38mpg actually on hills. taking it up takes only a small % more of fuel but on the way down its no fuel.

on highway I found when i drive 70-72mph I get 32mpg vs 28-29 at 80mph. On a big trip that's worth the savings

I overinflate tires too. worth it for savings and handling.

You made me think of this classic thread: [Fleabag's 'Inflate to Sidewall' discussion]

http://forums.anandtech.com/showthread.php?t=335140

 

[silicon]

i have a 2008 chevy aveo.

filled up the tank tonight. on 9.8 gallons i travelled 243 miles.

under 25 mpg in a car that looks like this:

now, i do live in a fairly hilly area.

but generally i drive conservatively i think. i always switch to the higher gear around 3000 rpm. i am light on the pedal. i do engine brake but in my research the fuel intake is shut off during engine braking and so should not use up any fuel.

so how is it that such a tiny low-powered car gets such bad gas mileage?

Have you elimi nated the possibility of a leak in the gas line or the gas tank?