I was told by a mechanic yesterday that the 1974 Vette I just purchased will need new ring pinion gears. It has been making a whining noise that is quite loud while driving and coming form the rear. It also makes a thunking sound when you take your foot off the accelerator. When I looked on corvette central they have 4 different sets ( all different rear ends , 3.08 n, 3.36 , 3.55 and 3.7) How do I know which one of these my vette would need. It has the stock 350 engine.

What transmission do you have?
To determine what the car had originally, assuming the gear ratio has not been changed, you need to check the code stamped on the bottom of the diff itself.

THIS pic will show you where to look.

THIS pic will beak down the codes you see. It's the first two letters that tell us what ratio it has...or had originally. However....the gears may have been changed at some point in the vehicle's life, so the only way to know for sure what ratio it has is to remove it and check...unless you know the history of the car, and know it is original.

Find out what the code is, and then refer to the following:

AA= 3.55
AB= 3.70
AC= 4.11
AW= 3.08
AX or LR= 3.36

awesome. My mechanic is removing the unit in a few days. I'll use this information to get the gear ratio. thanks

Question:

Is there any strong reason to stay with whatever gear ratio it has or would this simply be a good time to change it. The car will not be a daily driver nor will be be driven long distances. What would be the best gear ratio to use for in town limited use? The engine is a Q3 350-cid V8, 195-hp, Four-Barrel. I think it current has the 3.36 rear end.

The automatics used the 3.55 most often, the 4 speeds used the 3.08 or 3.36. Of course, the car could have been ordered with any of the above listed ratios. The 3.36 is the best all around ratio, imo. If you do a lot of highway trips/driving, the 3.08 would be best....it's really a matter of how you plan on using the car.