Just installed a new tach circuit board in the 84 z with a 350-crate motor-four speed auto- factory axle. The old one was why off for years so I'm not sure what to expect. I am now seeing 2000 rpm at 70 rpm, 2500 at 80 mph. Please verify these numbers with your experience. Thanks

 

You might calculate with gear ratio and tire size.

 

Are you talking speedometer here? That does differ with the taller tires 275s. The tach though gets a signal form the distributer, no? Really could change the gear on the cable if I could figure that. This tach is adjustable so looking for rpm/mph numbers from another driver with 84 z with gauges. thanks

 

Tachometer is independent of speedometer. It's reporting what is the engine rotating speed (rpm). Use a speed timing light to verify tachometer is accurate.

Speedometer is independent of tachometer. It's reporting vehicle ground speed (mph) and is calibrated with little gear sets in the transmission. Use a GPS to show your true speed and compare that to the speedometer.

When you get both calibrated, then you should be able to get on the highway with converter locked up and calculate mph from tachometer readout if you know your gear ratios and tire diameter, but it's not necessary because you already did the work to calibrate both gauges.