Thinking about downgrading to 3:42

[markd79ta]

and possibly just 1-3 MPG less on the highway.

The highway mileage was definitely less. ...28mpg vs. 22mpg. Actually, I was driving the car much faster with the 3.42s and the 28mpg average...over 90-95mph through the remote areas as opposed to setting the cruise at 85mph with the 4.10s. After you get above the 75mph mark, the 6th is more efficient with lower gears argument goes out the window. 6th with 3.42s at 75-80mph is a perfect cruising range and really good on the gas mileage. Since most of the interstates are 75mph out here, it worked pretty well with the stock gears.

Actually, and especially after this last trip, I am considering putting the stock gears back in the car as well. I kinda liked it better back then. The gears are great in town, but really **** on the highway compared to what the car was like stock.

[fastTa]

After you get above the 75mph mark, the 6th is more efficient with lower gears argument goes out the window.

Bad info here. With a 25.6" tire (275/40/17) at 75 MPH, 4.10:1 rear gear ratio, and having your t-56 in sixth gear, you are only turning 2013 RPM's. Even at 100 MPH with the same setup, you are only turning 2684 RPM's.

[markd79ta]

Bad info here. With a 25.6" tire (275/40/17) at 75 MPH, 4.10:1 rear gear ratio, and having your t-56 in sixth gear, you are only turning 2013 RPM's. Even at 100 MPH with the same setup, you are only turning 2684 RPM's.

What??? A 6mpg difference comparison on the same stretch of road for an approximate 1500 mile round trip is not bad information. Heck, the car had roughly the same interval of miles between spark plug changes with the exact same plugs used both times. We could discuss optimal RPM ranges on paper until we are blue in the face, but what matters the most is how the car performs on a road test. The LT1 turns all the RPMs it needs to obtain optimal fuel mileage at 75 in 6th gear with the stock gears and without bogging. By your own calculations, the 4.10s start pushing the RPMs up to almost 2,700 by 100mph. So, if you are turning the engine a lot less at say 85 with the 3.42s, I would venture to guess that this would consume less fuel as my car did. Anyhow, if this is bad info., then I have a pile of junk car.......because this is what it does proven time and again. If your car is somehow different, that's great.

[Jeff_in_Atl]

Using a 94-97 6sp for gears 4th=1.0, 5th=.74, 6th=.5

2.73 rear gears
4th=2.73 final drive
5th=2.02 final drive
6th=1.365 final drive

3.23 rear gears
4th=3.23 final drive
5th=2.39 final drive
6th=1.615 final drive

3.42 rear gears
4th=3.42 final drive
5th=2.53 final drive
6th=1.71 final drive

3.73 rear gears
4th=3.73 final drive
5th=2.76 final drive
6th=1.865 final drive

4.11 rear gears
4th=4.11 final drive
5th=3.04 final drive
6th=2.05 final drive

4.56 rear gears
4th=4.56 final drive
5th=3.37 final drive
6th=2.28 final drive

As you see, 4th gear with 3.23 gears is almost the same as 5th gear with a 4.56 rear end. You can see other comparable numbers.

Newton's law says every action has an opposite and equal reaction.

Bottom line. If the final drive ratio is exactly the same, the gas mileage will be exactly the same. Anything gained by a steeper trans gear is reduced by the smaller rear gear and vise versa.

Now if you throw in a set speed for comparison and the final drive ratios are different, yes there will be a difference in mileage.

Changing the cam will change at what RPM the engine is the most efficient. But that throws in another variable into the equation.

If you change the rear end ratio and you can now use a higher trans gear and the RPMs are the same, the gas mileage will not be any better or worse. If however the engine RPMs are higher or lower making it more efficient or less efficient, the mileage will get better or worse. I don't know when the LT1 is the most efficient. This more effects lower speed driving when you have the option of changing to a higher gear. At higher speeds this is not an option and most of the time a lower rear end gear will be more efficient.

I talked to a mechanical engineer about this and he is not positive but he really thinks that Newton's law applies here. BTW his last name is Newton too. LOL

[fastTa]

What??? A 6mpg difference comparison on the same stretch of road for an approximate 1500 mile round trip is not bad information. Heck, the car had roughly the same interval of miles between spark plug changes with the exact same plugs used both times. We could discuss optimal RPM ranges on paper until we are blue in the face, but what matters the most is how the car performs on a road test. The LT1 turns all the RPMs it needs to obtain optimal fuel mileage at 75 in 6th gear with the stock gears and without bogging. By your own calculations, the 4.10s start pushing the RPMs up to almost 2,700 by 100mph. So, if you are turning the engine a lot less at say 85 with the 3.42s, I would venture to guess that this would consume less fuel as my car did. Anyhow, if this is bad info., then I have a pile of junk car.......because this is what it does proven time and again. If your car is somehow different, that's great.

Your 6 MPG loss is on the extreme end of the scale pure and simple. Maybe you need to have your fuel system professionally cleaned, maybe you have a lead foot, maybe your engine is not tuned well, I don't know. What I do know is that a 6 MPG loss going from 3.42 gears to 4.10 gears with a t-56 is rare.

I went from 3.23's to 4.10's with a 4L60E in my 1997 TA when it was just a bolt on car and only lost about 2 MPG. I also drove the car considerably harder with the 4.10's. These results are consistent with most everyone I have talked to.

Hell, on my 1998 Tahoe, I recently went from the factory tire to a 35" tire and went from 3.23 gears to 4.56's front and back and I only lost 4 MPG. Went from 18 MPG to 14 MPG. And that is with a 35" mud tire.

[Joe 1320]

I went from 3.23's to 4.10's with a 4L60E in my 1997 TA when it was just a bolt on car and only lost about 2 MPG. I also drove the car considerably harder with the 4.10's. These results are consistent with most everyone I have talked to.

Well, I lost a little more than that going to 3.73s. I used to be able to squeeze out a max of 26-27 on the highway with 3.23s, now the most I can get is about 22 highway only. My average combined dropped from 22 to 18 mpg. I can't imagine only losing 2 mpg going to 4:10s with an automatic. A good friend of mine went from 2.73s to 3.42s and went from 29 mpg highway to 25 highway. That seems consistant as well. Maybe we're the oddballs.

[markd79ta]

Well, I lost a little more than that going to 3.73s. I used to be able to squeeze out a max of 26-27 on the highway with 3.23s, now the most I can get is about 22 highway only. My average combined dropped from 22 to 18 mpg. I can't imagine only losing 2 mpg going to 4:10s with an automatic. A good friend of mine went from 2.73s to 3.42s and went from 29 mpg highway to 25 highway. That seems consistant as well. Maybe we're the oddballs.

This sounds more in line with what I experienced. If I am really careful, and keep my cruise set at 75 on flat land, I may get 23 or 24mpg with the 4.10s. If you throw in mountain passes and/or another 5 or 10mph, it is 21,22. I always got between 27 and 29mpg with the stock gears.