Calibrating a cable driven speedo or Tach
by Larry Barickman
Since there is only one gear ratio for the cable driven tach on pre- 75 Corvettes and sometimes changing speedo gear is not always the answer for adjusting the speedo I came up with a method to calibrate your own tach/speedometer. This would be the case when the odometer is correct but the speed is reading low or high. Changing the gear in the trans WILL alter the odometer as well as the MPH reading. It is important to know if the odometer is correct before going into the speedometer. If your odometer is incorrect, you must first determine and install the correct gear in the transmission to achieve odometer accuracy. Also if you think your tach is wrong then you need to compare it to a known accurate electronic tach. You must know your rear end ratio and tire diameter. Assuming that your speedo or tach read high or low then read on.
The tach and speedometer are identical in operation and parts except the odometer on the speedometer. There are shops that will calibrate your units but they are hard to find and not cheap., and/or you have to send you parts out and wait and wait. Following is a method to do it yourself. When you open the unit you will see a small adjusting plate that has a VERY small watch-like mainspring hooked to it. Always scribe a line where the adjusting plate is now so you can return it to where it was. Now..since you have the dash apart here=s what to do.
First you need to check the tach. Attach the electronic tachometer, or tach-dwell meter, according to instructions enclosed with the instrument and temporarily put your tach on to the cable coming out of the firewall and start the engine, compare the reading at several RPMs. I choose 1000, 2000, and 3000. If your tach reads higher or lower than the electronic one, make very minute changes in the adjusting plate until you achieve accurate readings when comparing to the electronic tach. Be sure to recheck at all of the various RPMs. The more various readings, the better the result. Once you are satisfied with the tachometer, find a rear end ratio chart such as the one in Idavette.net or Tirerack. Put in the information so you will know what the speedometer should read at several RPMs; again I choose 1000, 2000, and 3000. Now, assuming that your odometer is correct, simply repeat the entire process with the speedometer in place of the tachometer. Use the electronic tachometer to determine the engine RPM and adjust the speedometer main spring tension until the desired mph correlate with the tach RPMs as per the chart. This may not be the most scientific method or approach but it works for me.