My 89 GTA has a 140 mph speedo that is reading approx 10 miles an hour faster than i'm actually going. i.e. says i'm doing 60 mph and i'm actually going 50 mph. Any ideas?

specs on car is it's a 89 GTA 5.7/auto

thanks for any help

 

Does the tire size match the size indicated on the sticker on the inside of the driver's door? I had a different tire size on a car once that caused the speedometer to be off 1mph for every 10mph I was going. Ex: If the speedometer said I was going 70mph, my actual speed was 63mph.

 

yes the tire size is correct

 

Has there been any drivetrain work done on the vehicle? Is the rear axle original? Has the transmission been rebuilt? There are a number of different drive and driven gear combinations available for the VSS drive. If work has been done on the trans, the wrong gear combination may have been installed. Look in the GM(Helm's) factory manual for your car to find the correct combination of gears to correct your speedo reading. If no work has been done on the car, you may have a defective VSS or circuit.

IIRC, on the 700R4 with electronic speedo, the VSS outputs an AC signal of varying strength(frequency and amplitude), which the speedo driver translates into a MPH value. A poor connection or interference in the circuit could cause a false signal. Make sure that no plug wires are laying against the main engine harness.