I just bought a 89 Iroc with a 700R-4 and a 9 bolt Borg Warner 2.77 posi rear and when i took off the differential cover I got a nice surprise 3 teeth missing off the ring gear. I decided that I am going to get a 28 spleen 10 bolt from a junk yard and rebuilt it with new gears. Here is my question what gears should I go to. If I switch to 3.2's how much gas mileage will I loose? How high can I go before I don't get any traction. I want to be able to stick almost as well as my 2.77's. Some people I have talked to say go to 3.7's but won't that make my car get horrible gas mileage and never be able to stick to the ground when I punch it?

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1989 Iroc-Z Blue T-Tops
Lightly Moded:
Headers, Rebuilt B&M 700R-4 tranny, Airfoil, K&N's

 

I went with 3.42's and it seemed to help my city fuel mileage. I just barely touch the gas pedal and the car goes. I probably lost a little mileage on the highway, but it's still pretty good.

 

I purchased my '86 Z28 new and it has 3.42 gears. The 305 TPI that was in it had more mods than yours and I thought the gear ratio was perfect for all-around use. The window sticker reads 25 mpg on the highway and that's approximately what I got with it. Now I have a 358 TPI in it and I am leaning towards the 3.73 gears for autocrossing. I recommend 3.42's for your application, you will love them!!

 

With overdrive, gears don't hurt mileage very much at all. 3.42's will hurt mileage very little. 3.23 probably won't change it at all.


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350 with stealth intake, holley carb, 470 lift cam. 700r4 with .5 boost valve, vette servo, tci lock-up kit, B&M megashifter. Richmond 3.73 gears, powertrax locker, timkin bearings, synthetic lube. Custom 3 inch single into 2 2.5 pipes. 1 1/2 drop springs, 1 5/16 solid front sway bar, 1 inch rear bar, custom subframe connectors, custom LCA relocation brackets. Kobel ground FX, currant red metallic paint. Lots of other stuff...
82camaro

 

But what about traction? If I put 3.42's am I going to loose traction easily? I have no idea about this because I have never changed the ratio before. In my other cars it’s always been good so I haven’t touched it but 2.77’s are too low. I am not driving on the autobahn (I wish). Any help would be greatly appreciated.

 

I'd go with 3:73's that's what I have and love them. You won't notice any great amount of lost traction, believe me you'll accelerate much quicker..