I have a 1980 rx7 with a sbc 350 with grannys kit and obviosly my tach is off, -it reads about 1800 when the timing light says roughly 900----so about double. Grannys says to pull the cluster and put in a resistor, but I can not get my hand in deep enough to remove the speedo. at least not w/o bvreaking stuff.

Grannys says:

"The method I use is to add a 1500 ohm resistor in parallel with the resistor BEFORE the cal-pot (usually marked R3). I usually add this resistor to the backside of the board (the side with the solder joints) to make things easier. The 1500 ohm resistor will get the calibration close enough that minor adjustment of the cal-pot is all that is needed."

I want to put the resistor at the tach signal wire under the hood. can you just wire a 1500 ohm resistor in line? is it the same as the ballast they sell with some coils?

 

Disconnect the speedo from the trans and any clips underneath the car. This will allow enough room to flip the cluster face down a little. Then disconnect the two round plugs and I think a couple smaller plugs. Be careful to not mess up the trip reset when flipping down. Also remove the little rubber piece on the end of the trip reset before doing anything. If it falls off it's hard to find. It's not a fun thing to take out.

 

Based on Grannys' advice, adding a 1500 ohm resistor in parallel with the existing resistor before the cal-pot should help calibrate your tachometer. If you want to place the resistor at the tach signal wire under the hood, wiring a 1500 ohm resistor in line could work, but it's essential to ensure it's in parallel with the existing resistor for accurate calibration. The ballast used with some coils serves a different purpose than the resistor for the tachometer calibration, so they are not the same thing. Make sure to follow the instructions carefully to avoid any issues with the calibration process.