-Q-

iTrader: (10)

Car is a '75 Corvette, and all the gauges are stock. The tach in a '75 is an electronic tach, so I should have been good to go by wiring in the wire from the PCM (PSI conversion harness) to my stock plug on the back of the tach, and then changing the high and low resolution on the tach output in HPTuners to 3 instead of 6...

The tach reads correctly at idle and up until about 2000rpms...then it takes off and is reading 500-600rpms too high by about 2300rpm or so (I'm comparing the reading on the tach to the reading on the scanner in HPTuners). The reading is off more and more the higher in the rpm range I rev the motor.

So from idle to 2000...correct.
Reading 2300 on HPT....almost 3000 on the gauge.
And so on...

Any ideas here? I just put a new circuit board in the tach, so that should be OK. I'm about ready to say screw it and put in a set of aftermarket gauges and be done with it, but I have everything else reading correctly except the speedo....

TIA.

1989GTA

Not sure in your situation if this is the problem but some recommend a 1K ohm resistor on the tach feed from the ECM with the other side of the resistor going to +12V.

tfi racing

iTrader: (2)

Might be as good as it gets,aren't those tachs notoriously inaccurate from the factory?

ls1nova71

iTrader: (4)

This would be my guess. A Gm tach from 1975 probably wasn't exactly super accurate in 1975.

-Q-

iTrader: (10)

Well if that's the case, then Autometer it is...