97RedT/AWS-6

The engine is more efficient at burning all the fuel in the combustion chamber at higher rpms which equals more power and less throttle.

shoebox

You have to consider engine load and not just rpm. We couldn't take a 2400 mile trip in first gear and expect to get good gas mileage.

You want to use the highest gear possible with the least engine load. The PCM uses the MAP sensor to sense engine load and the MAP sensor is just basically a vacuum gauge. Load is low when vacuum is high.

So, you don't want to lug it and you don't want to rev it too high for good mileage.

stereomandan

If you don't have your foot on the gas, or on the gas very lightly, do you have higher vacuum at 2500 RPM or 1500 RPM?

Dan

shoebox

Again, depends on engine load.

Anytime you can reduce the engine speed and the engine does not have to work hard to maintain that speed, you will get better gas mileage. Just look at how good mileage is with the M6 cars. The extra overdrive gear really helps. Even though the autos have less gears, their loss is not all attributed to [in]efficiency of the tranny.

Lt1 and Ls1

Glad you asked I have always wondered the same thing

Tim240z

I always worked off the theory that the engine is most efficient (which should therefore convert into most fuel efficient) when it (the engine) is at or very near it's torque peak. I've found that keeping the rpm in the 1900-2100 range, regardless of cruising, overtaking etc (which means appropriate gear selection) gives me the best fuel consumption (best of 24.8 mpg).

Tim

scoobysnax83

I'm glad I asked too.

95 Mystic TA

Good thread. I hear ya about the fuel economy; I'm a sucker for efficiency. But I've got one..

more fuel effecient,

at 70, more MPG's with t tops and/or windows down or up w/ A/C on?

OldSStroker

Listen to shoebox. He's right about this stuff.

On my A4 LS1 C5 the instantaneous fuel economy read out is interesting to watch. I've tried all the combinations. What gives best mpg is highest gear and lowest rpm with whatever throttle opening is required. The Vette A4 allows a lot of low rpm large throttle opening compared to other cars.

On a fairly steep hill at 50 mph if it will remain in 4th at 1400 or so even with plenty of throttle, mpg is 15-20% better than if it downshifts into 3rd and jumps to 2000 rpm with less throttle opening. Sure it feels "freer" and less loaded in 3rd, but uses more fuel.

Same thing when passing: 4th gear just crowding the lockup is considerably better than 3rd and less throttle even with 3rd lockup. This is nowhere near torque peak so VE ain't all that great, but there are other considerations.

Fricton hp is almost directly proportional to engine rpm. At 2000/70 mph cruise maybe 40 hp is needed to keep the car at 70 on a level road...maybe less. If fricton hp is 20 @ 2000 but jumps to maybe 30 at 2900 in 3rd, engine is producing 70 hp instead of 60 to get 40 to the flywheel. There goes 16% or so of your fuel economy. These numbers are not exact, but the principle applies.

My son's '00 Camaro SS (M6) on full tank 75 mph+ freeway trip of 500+ miles got about 2 mpg better than my A4 Vette, even though Vette has less aero drag. It was the 1.71:1 overall vs. the 2.21:1 overall ratio that did it. A4 was in lockup so trans efficiency wasn't much problem.

My $.02

94formulabz

A/C

95 Mystic TA

Damn, I guess after 2 summers, its time to get that fixed.

ImportKillerZ28

I always thought that the higher the RPM, the more gas you use, isnt that right? So if your on the freeway going 70, and you're in 6th, downshifting to 5th would give you tons of power and you'd pass em quicker, but it'd use more gas than accelerating slowly staying in 6th, right?! The guy that i bought my car from (a high performance sports car dealer) said that it is best to keep the Z28's around 2000 RPM's most of the time...he says that it is better to run it high because it is a high performance car. As if to say that they arent made to run at 1000 RPM's. I dunno, just wanted to see what others had to say about that...thanks!

OldSStroker

Your "high performance sports car dealer" probably had no part in DESIGNING the car. IMO, he's not an OEM automotive engineer, and is wrong.

You are more correct than that guy.

If you drive in 6th at 1200 rpm or so, your car will tell you how much throttle you can give it before it starts to complain. You will probably be surprised how well it runs at those rpms. If you need more passing power than you can get in 6th, especially when you get your foot to the floor, downshift a gear or two or three.

If you get a chance to drive a Z06, you'll find it likes 1000-1200 in 6th just fine, and keeps up with most traffic w/o downshifting.

My $.02

Mike454SS

I do agree with what you've said, so please don't take this as a newbie being a jackass (because I don't post here much, I'm usually on the Impala forums).

In my 96 LT1 Caprice, I got about 22 mpg cruising at 75-80 on the highway with stock 3.08's, the stock 4L60-E locked in 4th and a half assed cold air intake. When I blew up the automatic I converted the car to a T56 from a 96 TA.

Before I changed the gear ratio in the car, I actually always cruised in 5th on the highway, which resulted in a higher cruise RPM than the auto. ~2.13 final drive with the auto and ~2.81 in 5th with the T56. Despite this, my gas mileage on an otherwise unchanged car went up to 26 mpg consistently on the highway...cruising around 75-80 most of the time.

I then added a HOT cam and 1.6 roller rockers and a new catback to the car, and saw my gas mileage drop to 25 in the same conditions.

After this, I changed the rearend gears to 4.10's (8.5 inch rear end in the caprice). So now cruising in 6th my final drive is 2.05...sightly lower RPM than when I was completely stock with the auto. I still get 25 mpg cruising at the same speeds though.

So...what I am trying to say is I think that the locked up auto versus the M6 in terms or parasitic drag on the engine is still a decent difference that does provide noticeable differences in numbers. I say this because my cars aero drag has not changed at all obviously, and while the T56 is slightly lighter than the 4L60-E, the subwoofer in the trunk as well as a set of heavier seats I installed from a bonneville to replace the uncomfortable police car crap more than makes up that weight difference. I wish I had done the gear change before the cam/catback/roller rockers as a further point but I didn't get to, so sorry about that one discrepancy.

Brettinator

ummm...would that be just a chicken egg or do Ostrich eggs count ?

Anywho I don't think I'll ever see good gas mileage again ..and that really sucks with gas prices now.