To calibrate an electric speedo one places a prescribed frequency into it with a signal generator. The frequency is noted on the back of the speedo K or W equals xxxx. This frequency should produce 60mph.

Due to the different drive ratios of the differentials the numbers will be different from speedo to speedo.

There are folks who manufacture speed signal modifiers. The idea would be to take your speed input frequency and reduce it by 25%.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician