For my auto to manual swap, I bought a used set of gauges to replace the tach (7k vs 8k redline, remove PRNDL, remove HOLD) and modify my speedometer for the change from 3.9 (auto) to 4.1 (manual) gearing. Once you remove the speedometer from the instrument cluster, one side of the speedo has a small blue potentiometer which accepts a philips head or small flat blade screwdriver. Measuring the resistance between one side and the middle pin on the back of the circuit board, I took these measurements:

82.4 K ohms 5 speed with 4.1 gear ratio (4.1 turns of drive shaft to one turn of wheels)
86.9 K ohms automatic with 3.9 gear ratio (taller gearing where only 3.9 turns of drive shaft turns the rear wheels once)

I then carefully adjusted the potentiometer of my auto speed to match the 82.4 K ohms value of the 5 speed speedo. Moved it a very tiny fraction of 1/4 turn. At first I tried 1/4 turn and it went all the way to 20 K 0hms or 137 K ohms which was way off. Turn Counter clockwise to raise it and clockwise to reduce the resistance.

Lowering the gearing from 3.9 to 4.1 meant reducing resistance from 86.9 K ohms to 82.4 K ohms. So, if you had a manual already at 4.1 and you wanted to lower even further to say 4.5 ratio, you'd want the resistance even less than 82.4 K ohms.

Playing around with the numbers finds that the K ohms times the gearing is constant. 82.4 * 4.1 gives 337.84 and 86.9 * 3.9 gives 338.91 which is pretty close given the measurements varied a few tenths of an ohm some measurements.

I think you could take the constant value 338 and divide by the desired gearing 4.5 (for example) and you'd need to set the pot to 75.1 K Ohms for a 4.5 ratio.

You could test it by putting the cluster on the car and driving around and comparing with a GPS speedo before fully installing the cluster in the hood and putting the hood back on. Worst case just put it back, but I think this will work. At least my auto to manual should be accurate now.

Cheers,
Dan

 

 

Yes, just calculated 38.6K for 4.3. It took me 5 tries adjusting the pot to get it to 82.8K as it kept finding 83.7. Using the world's tiniest amount of adjustment I finally got it.

 

Here's my writeup on doing the speedo adjustment -

https://www.rx7club.com/3rd-generati...meter-1011051/

You can also just drive around with a GPS app on your phone and calibrate it to the GPS. My speedo is now dead-on accurate with 17" rims and a 4.44 rear end.

Dale