i have a 1986 trans am and the rear end went out . i purchased one from a 89 now my speed ometer is of 30 mph advanced and it drinking way more gas. do anyone know the original gear set up. so i can replace with the righ ring and pinion.

Unless your car has the G92 RPO code, a 1986 TransAm with a 305 and 700R4 will originally have 2.73 gears (highway gears)

The diff from an 89 could have 3.08 or 3.42 which will reduce your fuel mileage but will increase the performance of the car. That is providing those gears were original and the previous owner didn't install something else.

Pull the rear cover and count the teeth. On the ring gear should also be numbers saying how many teeth are on the ring and pinion.

thanks ill have it checked. so i can the proper ones. and get the speed odomer right

If you're willing to live with the fuel mileage because of the deeper gears, you can change the speedo drive gearing in the tranny to fix the speedo calibration.

once you figure out what gears you do have, bowtieoverdrives has a speedo gear calculator to tell you which gears you'll need to purchase to correct your speedo.