speedometer calibration

Have a pusher, allison tranny and speedometer is WAY off based on gps and measured mileage comparison. Have heard calibration is needed but have no idea who/where to look. I live in Florida.
speedometer calibration

My speedometer/odometer has only one button..to reset trip miles..it is mechanical there. Ever heard of missing/defective magnets in wheel sensor?
RE: speedometer calibration


My speedomter just went out.

It's a 1988 Rockwood on a Spartan chassis with a Allison 4 speed.

Spartan sent me Speedo Data Drive sheets (3). I don't yet know if it's mechanical or it sends a signal (rf)?? from tranny to speedomter.

Anybody have a experiences with Speedo's going out?

Oceanside, CA