In an effort to increase gas mileage, I have debated with this issue for some time. It was probably right after an episode of Top Gear or something, where they stated that you use no gas when coasting in gear opposed to using a small amount of gas putting the car in neutral (because the engine needs to keep itself idling) and coasting downhill, that I thought of this question:

Does it use less gas if I am...

A) Driving 50mph in 4th gear, RPMs at ~3300 and using LESS throttle

or

B) Driving 50phm in 5th gear, RPMs at ~2600 and using MORE throttle

To give some framework, I drive a 2000 2.5RS and get roughly 24.5mpg in mixed conditions, 27mpg straight freeway at ~75mph and 29mpg at ~65mph (which doesn't happen often).

Any thoughts on the topics from the technical folks or physics-inclined bunch?

 

Good question. I've often wondered this myself...

 

I'm curious too, is it better to shift early and have to use more pedal to go (higher gear lower rpm) or lower gear more pedal? Is there even a simple explaination?

 

*sorry- lower gear less pedal*

 

If you have a vacuume gauge, you can use that to help you decide which one will use more gas.

The rule of thumb is: More Air = More Gas

So...
Decide which gear to be in by looking at your vacuume gauge and choose whichever one creates more vacuume (use less air).

 

More revs = more friction, throttling, and pumping losses.

 

revs aren't free.
In theory, the best gas mileage can be achieved by short shifting WOT to the speed you want, then holding that speed very steady in as low of a gear as possible. Of course, this is impractical, no fun, and would lug most engines. But theories are like that.

 

I'm pretty curious about this myself, and I've got a digital A/F ratio gauge (ECUTek Dash Display) that seems very accurate. I've been driving for the past few weeks with one eye on it, and here are my totally non-scientific observations:

Gas usage is lowest at idle (car in neutral). Duh. The lowest usage of fuel is at 750 RPM with a warmed-up motor sitting at idle.

Shifting below 2500-2600 (bogging the car) uses MORE FUEL than shifting at 2600-3000. The ECU is most likely trying to get more torque at a low RPM and throws more fuel at it. So, myth that shifting at a low RPM is efficient is busted.

Coasting in neutral uses the same amount of fuel as sitting still in neutral. Coasting in gear (engine braking) uses more fuel than not (also awarded another "duh.") Second law of thermodynamics = no free lunch.

Pedal pressure while driving has a minimal effect on fuel use, as long as you aren't accelerating more than a few MPH. If the pedal dips lower than 1/2 pressure, suddenly the gauge skyrockets. Fuel usage (according to the gauge) can be SIXTEEN TIMES what you see at idle when you actually use boost. Using 1/3 pressure or less keeps you in a relatively lean use area.

The angle of the hill you are driving on has a VERY profound effect on fuel use. Even keeping the pedal in exactly the same spot, when going up a slight incline (I'm talking 2 or 3 degrees, barely enough to be seen/felt) the fuel use doubles or triples. Remember, we don't use a wire throttle in these cars; our fuel use is electronically controlled. Medium-sized hills eat fuel like crazy, almost as much as using boost. I tested this using cruise control, and I had the same result. 3300 pounds = nasty gravity/friction burden for the engine.

WOT boost is obviously (third "duh" award) the most fuel consumption, but surprisingly driving at 3600 RPM is just the same fuel use as driving at 2600 RPM. In other words, once you are cruising at a flat speed on a level road, RPM doesn't matter so much. Using boost, driving on a hill or bogging the engine all have a much more profound effect than just cruising at a fixed RPM.

If I was to list the biggest fuel-users in descending order, it would look like this:

boost/wot (greatest)
mild acceleration
driving on an incline
A/C turned on
cruising at fixed speed
coasting in gear
coasting in gear down hill
coasting in neutral
idle (least)

So, totally common sense, not the slightest surprise here. If you want to conserve gas in a turbo, AWD car, then I have the perfect driving method for you:

Drive it to a Honda dealership and trade it in.

Having a WRX and driving it with no boost, no A/C, light throttle pressure and coasting to every stop is utterly, totally, without question a complete castration of the car and the driver. Pay the damn gas money and smile, or sell that Subaru.

 

I have a ScanGauge and here's what I've noticed (only an opinion):

When cruising at constant speed on a flat straight path, you want to be in the LOWEST rpm's. I note that maintaining the same speed between two gears, the higher gear (and thus lower RPM's) have the lowest gas consumption.

When on that same path but you need to accelerate, you want to run the RPM's that is the highest operation efficiency of the system, which seems to be around 3k for most cars. If you look at a dyno, this tends to be where torque jumps very high. On an N/A car, this tends to be the torque peak. However, having a turbocharger complicates it.

At any rate, trying to achieve the same acceleration in the lower gear results in a very high load and gas consumption jumps yet you won't accelerate very fast... which (most importantly) means THE TIME IT TAKES TO ACCELERATE TO THE DESIRED SPEED IS MUCH LONGER. The lower gear (higher RPM) that operates in the optimal RPM range accelerates you quicker, and thus you don't spend as much time using fuel this way. Load is much lower. And then too high of RPMs is bad too because you either get into boost, or you operation efficiency goes down as evident in the reduced torque.

And don't forget that throttle position makes a huge difference. The operational efficiency range changes depending upon the rate of air you allow into the engine. I have no hard data, but it seems like lower throttle move the operational eff range down.

So, I found that the best way to efficiently use fuel is:
- cruise in as high gear possible
- when acceleration is needed, down shift as necessary to stay in the operational efficiency range. For granny accel, that's about 2.5k rpm shift point, for moderate accel it's about 3k, etc.

I think the good thing is, torque is something your butt dyno actually feels. Pick a throttle and hold it. As the RPM's rise, you'll feel where it seems to be working the best. You want to stay in that range so choose gears appropriately.

 

While commuting in typical California traffic, I found that the wake of an SUV (and van) is about a car length and a half behind, where instantaneous MPG jumps like 30-50%. You don't need to tail gate, but you are just a car length or two behind. The wake of course changes depending upon the car's size and shape. Big trucks have HUGE wakes (like 3 car length)

Forget drafting sports cars. The wake is so small you have to crawl up their *** which is dangerous and will **** them off.

The cool thing is, you don't need a scangauge to know. Just creep up behind from several car length away until the butt dyno feels a slight "relief" to your car working to accelerate. Just before you enter it, you'll feel a slightly harder time for the car to get in, which is probably the turbulance region where the wake is collapsing. You pretty much just have to get the nose of your car into the wake to continue the stream.

 

Yeah, you pretty much said exactly what I said :-)

Good to see someone else putting some actual numbers to this; I think we've put the "WOT until you reach speed" myth to bed permanently.

Oh, and the drafting thing is a good idea, but I can't do it consistently enough to really see a MPG difference. My favorite for drafting is UPS trucks -- they speed, and they use the radio, so you can be certain that there are no speed traps in the area...

 

Great information, thanks. I did see on a show one time they said that it is better to coast downhill in gear rather than in neutral. The justification was that when you are in neutral, your car keeps the car at from stalling by providing gas and air to, well, essentially "idle". But, when you are in gear going downhill with no gas, gravity is fighting you car's urge to stall because there is no gas/air being provided - thus, using no gas. It was a TV show, but it sounded like it made sense.

 

Only thing is, that's totally wrong. The theory, that is.

I used the scangauge to dispell this myth. Here's the deal. When coasting downhill in gear (and no throttle applied by foot), there is still the idle throttle setting. If the throttle closed completely, you'd probably bust your throttle plate (or something) as the engine sucks really hard as it tries to turn.

The thing is, the ECM has no idea that you are coasting downhill or when the momentum to the wheels will stop. So, with no throttle, you are at idle throttle and the higher RPMs of the coasting uses more fuel than the neutral idle (~0.9k RPMs). I watched the GPH (gallons per hour) parameter and can see that neutral idle is ~.3 GPH wheras in gear coasting is proportional to the RPMs the engine currently is at.

One more thing, with no fuel burning, your cat would also cool down and not be able to do it's job.

All facts point to idle fuel level constantly being injected, even coasting downhill

MYTH BUSTED

 

chimchimm5 rocks my socks

 

Agreed...

Oh... and more data.. in low RPMs if you accelerate and lug the engine (high load %), the fuel system quickly goes OPEN LOOP. Whenever the load is high, it goes open loop, and dumps fuel to make rich mix, probably to reduce the detonation.

At any rate, you want to stay CLOSED LOOP for most efficient operation.

ScanGauge is a COOL thing. BTW, I have one for sale because I'm upgrading to the newer version. New version is $160. The 1st gen I have has recently been reflashed to the latest firmware and a new and improved OBD2 cable added to it; all by Linear Logic themselves (I sent it in). Asking $90 + shipping. PM me if interested. www.scangauge.com