How much error is there in the speedometer reading vs. actual speed, in what direction--does the speedometer read fast or slow?
Is it a fixed mph amount (ex. always +5 mph), or a percentage (ex. always fast by 10%)?
You can verify the operation of the speedometer gauge by using the instrument cluster diagnostic mode to observe vehicle speed digitally on the odometer--the analog and digital displays should be very close, withing +/- 2 mph. If not the speedometer gauge may be bad, or the indicator placement is incorrect. Here is how to access and use the cluster diagnostic mode
If the analog and digital displays match then there are just two common possibilities:
The final drive gears are not the stock 3.27:1 ratio;
The tires are not the same outside diameter as the stock 245/45-17 tires (25.6") ;
Another possibility is that the PCM has a custom tune that was modified for some final drive ratio other than that actually installed.
A final possibility, in which the speedometer would be "way off", would be that the transmission had been changed from an auto to a manual or vice versa. The auto tranny has a 6 notch reluctor ring, the manual a 12 notch ring. The manual therefore produces twice as many pulses per mile...
- 2003 GT, UPR X, FRPP 24lb/h, Magnaflow, PP 70mm TB & plenum, Delta Force tuned,
Steeda UDPs, Ralco flywheel, RAM HDX clutch, 3.73s, 262 rwHP/305 lb-ft.
New ride (7/1/2013) 1998 Mercedes SL500-5.0L 32V VVT 326/347 HP/tq