I just went from having 2.73 gears to having 3.73 gears in my 89 trans am. The car will shift fine at low speeds, but when it kicks down into 2nd when the gas pedal is to the floor the car does not want to shift into 3rd and the runs in red line for the RPM's. But when you let off the gas a little the car will shift. We have a hypertech chip and a shift kit in the car already. The speedometer is off because of the new gears. Before the new gears were installed the car shifted fine at all speeds. Any suggestions on why this might be happening?

 

It sounds like the TV cable is not adjusted properly. But changing the rear end gears wouldnt change those shift points. But being that its different gears the car is acting different. Id have the cable adjusted and see if the car shifts better. You can do this yourself do a search on TV cable adjustment or take it to your local tranny shop and they should be able to adjust it for you for a small fee.