My 89 Buick Skylark speedometer reads 10 mph fast.

problem VSS sensor
wrong size tires fitted
wrong size rims fitted
mostlikely it is the VSS

check rim sizes and tires fitted as they may be smaller than the speedo sender is gear ed for
smaller dia. tires = faster speedo reading as the speedo reads off drive shaft speed

Make sure you have the correct tire size on the car. Door jam sticker should show the correct tire size.

It probably has something to do with the tires. Does it have the original size tires on the truck? If so then maybe someone had changed the speedo gear when they had bigger or smaller size tire on the truck.

Tire size. A change from original equipment size to larger or smaller for each size change will increase or decrease the speedometer reading by approximately 5%.

Good observation. Changing the tire size increases or decreases the number of wheel spins per mile, and that changes the speedometer reading. GM or a custom truck shop should be able to modify the speedometer drive, but most drivers just note the inaccuracy and learn to live with it.

Bill B

A vehicle speed sensor that is starting to fail could cause this, or less than likely bad wiring or connections, or yes, a bad cluster could do this... But how exactly did she notice the speedometer was 10MPH fast?? There are no real accurate speedometers in vehicles (Don't get me wrong they shouldn't be off by 10MPH) but there is a 5 or so percent difference high or low in many vehicles even with stock powertrains. I've plugged in with a diagnostic scanner on many vehicles and the dash speedo shows 60MPH, but the electronic speed readout on the scanner reads as low as 55MPH or as high as 65MPH

most certainly a speed sensor problem!
it may not be pulsing properly or the drive gear is worn etc etc.
Cheers Rob

someone has modified. the speedometer cable has to be taken loose from the tranny and put back in. the only way it can be that wrong.

Have you called transmission place? Did they also install it? Ask them if they changed speedometer adapter, and mention the difference in speed.