It’s been a while since I last posted so first off, I want to say hello to everyone and I hope everyone has been doing well. I just bought another 1976 Corvette with a L48 350ci and an automatic 350 turbo transmission. This car is almost all original, but the previous owner told me he had the front and rear suspension rebuilt, which I confirmed that it is all new. I also noticed that the drive shaft and both half shafts have been replaced. Here’s my issue, my speedometer is reading right at 10mph faster then I am actually going (For example when my speedo is reading 65, I am actually only going 55). I took the speedo gear out of my other 1976 Corvette with a known accurate reading speedo and it was the same color (reddish orange). I put it in anyway and it has the same problem. I noticed that the rear differential cover looks as if it has been removed, which tells me that the previous owner may have also changed out the rear end gears (stock gears should be 3.08). I know that you can lift the rear of the car up and turn the drive shaft manually and count the rotations of the tires and get a idea of what gears might be in the rear end. I was just wondering if anyone might know of a better way of getting the rear end gear ratio without having to take the rear end cover off. The tires are stock size. Thank you for all of your help, Dan.

 

That's the only way I know of. Appears that a PO has put something like 3.55 gears in it. I have 3.55 gears and 255/60-15 tires and it runs about 22 mph/1,000 rpm.

 

The speedo reading 65 when your doing 55 is about 10% off. Going from 3:08 to 3:36 is about 10%. Jack up the rear and have someone spin a tire and count how many times the drive shaft turns. If it's 3 and 1/3 to 1 you have 3:36 gears.
You are on the right track changing the transmission gear but you need the correct drive and driven gear set. Here's a chart from my 74. PG.

 

well since you have figured out that your speedo is about 10MPH off you can use this calculator to get and idea of what your rear end ratio is.
http://www.richmondgear.com/101032.html