i have a 87 GTA 305 5.0 tuned port injection, t5 tranny i think i got a dyno tune also, cuz it was my dads godfathers practice car for circut racing, but any ways yea i wanna have a good gearing for the track and mpg doesnt matter, i heard 3.73"s was too much for my 305 any suggestions?

 

Which rear end is in this car? Is it the GM 10 bolt or the Borg Warner 9 bolt?

 

i think its a 8.5 bolt its a 87 trans am GTA 5.0

 

There were only 2 rear ends available in that car. One is the 7.5 10 bolt and the other is the BW 9 bolt. The 8.5 10 bolt was not available.

 

ok yea i was off its a 7.5 10 bolt

 

What gears are in it now?

 

not sure there stock 3.08"s i think am not sure at all thoa

 

If you have 3.08 to 2.14 gears then you will have a 2 series differential and you will need the special thick gears to fit it. If you have 3.23 to 4.10 gears you will have the 3 series differential and you can use the standard thickness gears. If you know that the rear end is original and has not been changed then you can check the RPO code on the sticker and it will tell you what gear is in it. This sticker will be on the bottom of the console lid or on the back of the spare tire cover. The rear end codes start with G. A 3.08 would be GU4 and the limited slip differential woudl be G80.

A 3.42 gear would make a pretty big difference in the performance of the car. I don't know enough about the tuned port engines to say if a 3.73 would be too much or not.

 

thanks i will be checking that out to see what i have am sure its original not changed and thanks for the info

 

a 3.73 would be fine/fun with the t5. I wouldn't keep the 2 series carrier though, and you probably have the 26 spline axles. The best place to start with would be to find a 88-92 rear with 3.42(preferred) or 3.23 gears. They have the stronger 28 spline axles and would be a 3 series carrier. Swap the rears and if you are lucky enough to find a 3.42 rear you may be happy with just that.

-Jim

 

You would need a '90 to '92 rear end to get the 28 spline axles. You would also get the good PBR disc brakes.

 

The powerband of the engine is choked by the TPI style intake. That said, if you're going to stay with that intake for a while, go 3.42.

Yes, 3.73 gears work fine with it, but you don't gain much in the way of speed or economy going with them.

 

yea i ended up going with 3.42 oem gm rear gears it has a 27spline and says it will fit any 2 series or 3 series diffy's so hopefully it works good thanks

 

It won't... one or the other, but not both. (by itself anyway)

Hopefully you won't find out the hard way that those are REALLY 3 series gears and fail to mesh by around 3/8" on your most likely 2 series carrier.

IMO 3.42 is too much for a stock 305 TPI; good for one with exhaust upgrades (headers) but will force a stock one up into too high RPMs too fast. You won't be able to shift quick enough to keep it from running out of breath. Consider a COMPLETE exhaust system, heads to bumper, as your next mod. Assuming of course that the gears work for you at all.

 

this shop that i ordered the gears from said it would but if anything ill swap the rear ends with my bro in laws 90 camaro and give him mney and my rear end, and his rear is posi, also am gonna run a 2030 crane cam so it will extend my rpms a lil bit more, but i know the tpi"s run out of breath

 

Sofa is right, a gear will only fit a 2 series or a 3 series. It can not fit both. Someone told you a lie or you misunderstood what they said. All of the GM 3.42 gears are made for the 3 series differential. If you currently have a 3.08 then you have a 2 series differential. It aint gona work.

I have a feeling this isn't going to have a happy ending.

 

Had a feeling it was too good to be true but oh well ill just order the bearing spacer to make it fit, good thing I didn't buy the gears new, thanks guys u helped me from making a big mistake

 

You mean a ring gear spacer? That's another bad idea. If I still had my webshots account I'd show you some pictures of exactly why it's a bad idea. Why don't you just get the correct parts and do it right the first time so you don't have to go back and do it right the second time?

 

Dammit lol 3rd gens are a headache and expensive now

 

Mine requires blood and sweat and hard earned money but the more time and effort I put into it the more I like the car. There's a bond created so in the end its all worth it to me. You may find the same with yours.

 

I just swapped my 3.08's with 3.27's (9bolt) and I feel that is just right. IMO going to the 3.42 or 3.45 would of have been to much for all around driving. If you will be using the car mostly for the track them I would guess the 3.42 or 3.45 would of been just fine. After all GM did use that combo.