For some reason my speedo is off by about 8 mph on my 89 Vert (says its faster then it is). Is there a way to adjust it?

Thanks

 

If your tires are not stock, the tire circumference may be wrong. This will make the speedo off. There may be other ways to calibrate it, but the only way I know is to either get the right size tires, or have a mechanic re-calibrate the speedo.

 

The car is a vert and has s5 turbo wheels on it. Would that make that big of a difference?

 

There is calculators for this kind of thing, search google for tire size calculator or something to that effect.

 

The reading is off by 8MPH at what speed?

Speedo error is normally expressed as a percentage- i.e. "5% fast at 60MPH".

Since the speedometer is a purely mechanical device, correcting it is a purely mechanical procedure.
You can either change the speedo drive gear in the transmission or , more commonly, a corrective gearbox is interposed between the cable and the gauge itself (this is how "calibrated" police speedos used to be done).