Rear End Ratio question

Here is the math;

(RPM x TC)/ 1056 x R1 xR2 = MPH
where TC is tire circumference, and the Rs are gear ratios;R1 the diff, and R2 the tranny
So for you (3000 x 80.8)/1056 x 3.54 x 1 = 64.8 mph. The error factor in your reading would be due to TC slip, and/or calibration.

64.8/60 =8% slip/error , Very close to right on, Id say.