I have an 82 Camaro Z28 with Automatic transmission, which I thought to have 2.73 gears. Today I went to the MPH Calculator and entered 13.05 for the radius, (7.5 (15" Wheels / 2) + (9.25" * .6)" (P235/60 tires) / 2), 3000 for RPM, 2.73 for Gear Ratio and 1 for Tranny Ratio (3rd Gear - TH-200C).

The answer it gave me was ~85 MPH.

At 3000 RPM I am only at ~65 MPH.

The car had 1 previous owner, and I'm thinking maybe he changed to 3.73 Gears? This was the only way for me to come out close to 65 MPH at 3K RPM.

Does my math and everything else work out right???

 

There is a super easy way to check rear gear ratio. Jack up the rear. Put a mark on the driveshaft and a find a reference point on the rear end housing. Do the same for one tire. Turn the tire over until you can get a whole number for shaft revolutions and tire revolutions. Divide shaft by tire and get the rear ratio. If he did switch ratios, be happy that he changed the speedo gear too. I have to guess how fast I'm going, because my speedo got "wounded" in a install mishap.

 

You're over-estimating your tire radius, and the rest is speedo / tach error. I know what the math works out to, but thats assuming your car isn't sitting on the tires.

Take a tape measure and measure the distance from the ground to the center of wheel. That is your radius.

You're new answer will probably be a little closer to 80mph at 3k rpms. Beyond that, it's easy to believe 2-500rpms of tach error, or even more than that sometimes. And factory speedos can be just as bad, especially over time. The speedo in my old 85 Z28 was calibrated perfectly (mile markers and Odo would match damn close on long trips), but the speedo read 10mph slow at 70mph. Never knew until one time i had a buddy following me on a long drive, and when we got where we were going he remarked at how crazy i was to never dip below 80, and my cruise control was set at just over 70 the whole time.