It's also possible that some of your wiring work has gotten the speedometer ratio shifter out of phase with the rear end. When the rear axle shifts ranges, there's also a shift made at the speedo drive cable. I'd suggest using some kind of GPS unit to confirm that the speedo indication is accurate, although the engine RPM at highway speed should give you a clue. Is the governor still working?
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!