ok guys i know this has prob been beat to death but im thinking of changing my 92z vert auto gears to either 3.23 or 3.42 the problem is with the 2.73 gears now and 245/50/16 the tach shows 2250 at 70 and the calculator at wallace racing says it should be 1749 and 2191 with 3.42 gears also my 94 c1500 with 3.42 gears and auto shows 2400 at 70 with 275/60/15 and the calculator shows it should be 2011. both tachs shouldnt be wrong so what am i missing. thanks in advance for the help

 

The car is old.
Parts get changed.

Your diff. / ring and pinion could have been changed. A fluid change with gasket is cheap, and you could count the teeth to be sure.

The speedometer gears that drive the VSS could have been changed. Those are pretty easy to inspect and count.

Then, you'll know whether your tach is off.

 

nothing has been changed its been in the family since it was new and i hooked anther tach up to double check and it reads the same. thats what gets me is i know the car and also my truck is showing diff from the calculator also

 

Your speedometer could be off. Torque converter could be slipping. You don't have what you think you have. You can't do math. All these are possibilities.

Break the problem down into every little piece that could effect the results, verify each piece and you'll find the discrepancy.

 

I understand what your saying but I have used gps for speed on both and used another tach on the camaro and punched in on the calculator .7 for the overdrive gear so I'm not sure what the issue is. Does anyone else use the calculator at Wallaceracing.com

 

Is your tire size stock? it should be a 26" tall tire.

 

25.7