[Big Mike41]

Hello guys, I have a 1998 camaro Z28 with an automatic and a 2.73 gear. I want to swap to a 3.73 gear to add more performance. Can anyone tell me what type of gas milage I can expect, and what my rpm's will be running at 70 mph?

[Steiner]

Originally Posted by Big Mike41

Hello guys, I have a 1998 camaro Z28 with an automatic and a 2.73 gear. I want to swap to a 3.73 gear to add more performance. Can anyone tell me what type of gas milage I can expect, and what my rpm's will be running at 70 mph?

(Current RPM @ 70 X 3.73) / 2.73
Example: (1800 RPM X 3.73) / 2.73 = 2460 RPM with new gears

That's the quick way. There are several online calculators where you can type in your trans ratios and tire size to get a lot more info.

I had the same setup in an '01 model. With the automatic, you may be happier with the 3.42 if you do a lot of highway driving although I believe the auto SS models came with a 3.73. Keep in mind that you cannot just change the gears. You are going to need a series 3 carrier. Your carrier is a series 2 and only good up to a 3.08. I have a '97 supercharged S10 five speed and went from an open 3.08 to a Torsen 3.42 from an LS1 Camaro since they use the same rear end, and I wish I had only gone to a 3.23 when I do highway driving. I mainly wanted the Torsen but ran across a cheap set of 3.42 gears.

Hit eBay and look for a series 3 Torsen pull-out, a set of used gears, and with labor and rebuild kit you're going to wind up spending at least $600-$700 for the whole she-bang unless you can find a good shop with low hourly rates. Those are getting few and far between.

[South Side Goons & Hitmen]

I copied & pasted some of what I posted on another website earlier this year----I would also NOT recommend using any rear end gears numerically larger than 3:73. The 3.06 first gear of the 700R4 is the most aggressive of all the GM transmissions and anything larger than 3:73 will incinerate the rear tires. FWIW GM never offered anything larger than 3:42, 3:43 or 3:45 Numerically depending on where the rear end was made from 1987 onward once the 350 engine came back into production Camaro's for the 1987 model year. After 1981 until 1987, the 350 engine went on hiatus. GM did this because the 700R4/4L60/4L60E/4L65E has the widest spacing of any GM automatic transmission during the 1-2 shift and the torque multiplication factor with 3:73gears or larger just incinerated the rear tires......You see a 3:42 rear gear behind a 700R4 is like a 4:00 gear behind a turbo 400...Put a 4:10 behind a 700R4 and it acts essentially like a 4:68 gear...Problem is the gear spacing on a 700R4 is 3.06, 1.63, 1:1 and .70....The 1-2 shift lags which is why as mentioned above GM NEVER offered anything higher than 3:42 or 3:45 rear end gears for 3rd gens once the 1987 model year began. Trust me on this---see car #2 in my signature below!! It had a 700R4 in it and so did car #3.....

As far as RPM's, enter your overall tire diameter, MPH and .70 overdrive ratio here in the final box near the bottom http://www.csgnetwork.com/multirpmcalc.html


***Yes, you will also need to upgrade your 2 series carrier to a 3 series carrier as well. Here are the gear ratio's available for your car http://www.camarosource.ca/main_new_...or%20my%20car?

Here is some more info "On cars with automatic trannys and the 2.73 rear (GU2) 1st gear can't be selected because in order to pass the Federal noise pollution tests, GM had to lock out selectable 1st gear. It has to do with downshifting into the lowest selectable gear at some speed as the car goes through the sound traps. With the 3.23 rear end, the required speed is high enough that the tranny won't allow a manual downshift all the way to 1st gear. But with the 2.73 gears, the car will downshift all the way to 1st when manually selected thereby increasing engine and exhaust noise. So GM had to prevent 1st gear from being selected on the 2.73 cars in order to meet the guidelines. However, making 1st gear selectable on these cars is relatively easy. Lift off shifter cover, locate plastic (nylon) stop at the right side of the notched selector ramp, use a hacksaw or X-acto blade to remove the offending (square) stop. Then you will have first gear." http://www.camarosource.ca/main_new_...other%20gears?

FWIW, A friend of mine who is a Master Certified ASE Technician races some 4th gens he has....I'm not into 4th gens as I think they look like Geo Storms on steroids as one automotive press reviewer called them back in 1993 but I am learning more & more about their technology from him and from research...

I hope this info helps you out!! I would go w/ the 3:42 gears & a 3 series carrier and absolutely no larger than a 3:73........Most 3rd gen's had 2:77's & the G80 rear end code..I know yours is a 4th gen but the gear ratio's of teh automatic trans are the same...

P.S. A turbo 400 trans has a 2:48 first gear which is less aggressive than the 3.06 first gear in a 700R4/4L60/4L60E/4L65E family.

[SPARKY69]

I got a 00 with a texas speed 406 and a 410 rear!! Love it!!

[ace's68]

A friend has an 01' z/28 with a cam, full exhaust & longtubes, tune, numerous suspension mods, M/T rears and 4.10's, it runs an 11 something.
When he broke the first set of 4.10's he put 3.73's back in it and he wanted to shoot himself, eventhough it was a small change he said the car lacked a lot.
I would say just do the 4.10's.

[junior28570]

I ran 3:73's in my 02 WS6. 408ci making 486rwhp through a 4L60E w/ 3600 stall. I got 18mpg hwy. with the AC on. Car ran high 10's on the motor. It's all in the tune

Most guys in the 4th gen group will run 4:10's with the T56 trans though. Because of the 6th gear, they see a very minimal drop in mpg.

[junior28570]

BTW .. this should help answer a lot of your gear / tire / mph / rpm questions

http://www.f-body.org/gears/

Mileage will mostly be depedant on your tune and driving habits