I am changing the 4 spd in my 77 to a 5 spd. I am putting in a 5 spd that is marked from the wrecking yard " 81 to 83 ZX ". The rear end in my 77 is stock whatever that is. I know there are several types of rear ends. Should I change the rear end to be copatible with the transmission and how do I tell the diferrence between rear ends. I have several that I can choose from but don't know which is which.

The differential in your car should be an R200 with 3.54 gears.

The only way to truly know the differential gearing is to remove the rear cover and look for a stamp on the ring gear. The stamp contains two numbers offset by a colon like 37:11; divide the larger number by the smaller to get the gear ratio. In this instance, 37/11 = 3.36. I have a 39/11 or 3.54 differential in my car.

I am at work and do not have access to the transmission calculator, but you will most likely want to change out the differential to a 3.90 if you use an 81 to 83 zx transmission. The 81 to 83 transmissions had a taller first gear and were mated to a 3.90 differential from the factory. It really depends on how you drive though.