My speedometer is off about 10mph at 50 its reading 60. I have the PCM tuned with 245 50R16 and 3:42 gears which should be all standard from the factory. Those are the wheels/tires I have on the car. There's no reason to believe that the gears are different because when I bought the car it didn't have any upgrades. Can the speedometer lose accuracy over time/mileage?

 

If you have a way to read the speed from the PCM using a scanner, you can compare the PCM and speedometer readings to see if they agree. That would point you toward the problem. The programming doesn’t age, but the speedo can. How accurate is the odometer?

You could also compare the speedo reading vs. actual over a wide range of MPH to see if the error is a fixed number or a percentage.

As an outside cause, high voltage interference with the VSS wiring could affect the signal to the PCM, but the VSS wiring isn’t near any high voltage wiring.

 

Coldcuts,

Really, for accurate speeds, at any speed, beg borrow or steal a GPS speedometer. ~US$ 30.00 at Amazon, just feed it with a USB-mini cord, done deal. When my Blazer speedo went out, I got one to stay inside the speed limits; they are great to have in the school zones. Never bothered with fixing the OEM Blazer speedo.
That gadget will give you good info for troubleshooting there.

 

Speedometer miscalibration can affect the 4L60E transmission performance, cruise control and traction control. Even affects the Skip Shift function in the T56, although most people disable that.

 

I've been using the GPS speedo - mounted it in front of the real one. Just doesn't look that good. 😕