[TBONE1964]

Originally Posted by Already Gone

I hear what you are saying and its funny you picked those two tire sizes because I used to have 235/70 15's on the truck and now have 255/70's and the tach is out the same amount with both tire sizes. Could it be the tach is pooched or because the factory tach is hooked to a HEI which it was when I bought the truck.

I have never heard of anyone having a problem like that running HEI. You should get the same pulse from an HEI coil as you would a point style coil.

If you can find someone with a timing light that reads RPM's, it will be a good reference against your tach. It is possible your tach needs to be calibrated.