Speedometer is Too Fast

The sending unit for that truck is probably defective.

http://www.therangerstation.com/tech_library/speedometer.htm

Nice link, I just learned something.

FIRST check mile counter accuracy at a known distance. If it's OK and 10mph is a constant error in whole range, definitely meter problem, just disassemble and twist lever by 10mph. (You could also mess with biasing on it's Freq to Voltage converter if it's an analog meter). If error is a function of speed AND meter is analog, one could mess with converter's gain. If it's full digital stepper controlled you can safely dispare.

following a link on kramarat's link takes you to an area that discusses recalibration of the system. it is entirely possible that the gear ratio in the rear differential has been changed throwing off the calibration.

During that year some of the american made autos were produced with wheel

equipment that was set for 55 mph as the constant speed.Could that have worn down

the mph detection system after driving alot faster for a long time.

um, no.

Before doing anything, please tell us how you know that the vehicle is showing a 10 MPH discrepancy? The method used may be at fault.

My personal method is to use a GPS with a MPH display and drive on a road that is mainly straight or only very long slight corners. Sharp corners will ruin the accuracy. A straight road is the best for testing!!!

Generally speaking a speedo may show a higher speed than actually being achieved, but never a lower one.

Also maybe your vehicle was originally sold with smaller than standard tyres/wheels.....its possible!!!

i went through a road side "your speeds is" set up more than once and my speedometer always reads 10 mile an hour slow. wether it is on the hi way or in town.

Also i have had people follow me with the same results.

of course those "your speed is" things are probably calibrated on the low side too... they have an agenda to make you drive slow.

Borrow a GPS and find out exactly, then get a workshop to adjust it correctly....

Lyn -Door Industries sells peel and stick numbers to recalibrate your speedometer.

Hi guys... some instruments have settings behind the speedo to set up for different wheel sizes or differential ratios etc;.. I would 'assume' it is an electrical system & not mechanical.. also some speedo sender units (down on the transmission) are adjustable with letters/numbers in relation to wheel & diff ratios as well.. check this first.. Is easier than removing instrument cluster.. less swearing involved .heehee....

cheers for now...

A.J