Riddle me this...

Too many variables.
The tach could be out, or the speed-O. The rearend has not been proven, and the tire measurement has to be a roll-out measurement.
And the final variable would be the TC.
If the tire roll out is 77 inches, and the gear is a 2.76, then; the math says 65=2460 at zeroslip.
If the rpm of 3000 is correct, then the speed has to be 79mph with a 2.76 to satisfy zero slip.
If the roll-out is 77 and the gear truly is a 2.76, and both the tach and speed-O are correct, then the TC is slipping 540rpm or 22% which would be outrageous. I would expect 5% to be reasonable.
You can prove the speed-O by the fact that it takes exactly 1 minute to drive exactly 1 mile at 60 miles per hour. Run a 5-mile stretch and do the math. The percentage error in math equals the speed-O percentage error.