I'm swapping the engine/tranny from an '89 Firebird into an old Datsun Z. The donor car used an electric speedometer, which I'm going to use in place of the original unit (with a whole bunch of super clever surgery and a laserprinter). Because the Z will use "shorter" tires, I'll need to recalibrate the thing, but after taking it apart I'll be darned if I can figure out how. There seems to be no trimming pots anywhere in the instrument itself, or the VSS buffer. So here's the idea: because the tires will be shorter, and therefore turning faster at a given speed, I believe I can put a variable resistor in the hot lead running to the instrument to lower the voltage a bit and cause the needle to read a bit lower. Is this a workable idea? I'm thinking I can run the pot someplace under the dash, and adjust it by, like, following school busses, or comparing the numbers on the tickets to the speedo reading, or something.

 

I just changed my speedo in my 87 trans am 85mph cluster - 140mph cluster and i was wondering if it would be out of calibration at all, if so i do i get it calibrated. Do it yourself, or mechanic, if so about how much?