I have a stock (far as I know stock) 87 Iroc L98 with 255/50/R16 tires and my speed seems to be an even 5 over what it reads, 50mph is really 55mph. I checked this with several different gps speed apps several different times. How could I go about correcting this? Being 255 wouldn't make that much difference over the stock 245 size would it?

 

On each wheel rotation you cover more distance. It means the speedometer is off showing slower speed than real. A 255/50/R16 is around 26.1 diameter & a 245/50/R16 is around 25.7. You cover 1.5% more distance per rotation. So your real speed should be 51mph when speedometer indicates 50mph. 5mph over at 50mp/h is 10% error. That is a lot.


Best of luck!

 

Well since tires couldn't cause it and my differential looks crummy and un-touch is their a way to check the cluster to see if it's been swapped or possibly adjust the cluster to read correctly?

 

What was the original size of your tires? I assumed they were 245/50R16! Check on the door sticker. I don't have an easy fix for that. If all stock, maybe the speedometer is just getting old. I had to restore my cluster after 23 years! All gauges were off. If it was modified or repaired, heck knows.

 

Yes the original size was 245/50/R16. Also thanks for all the information.

 

If the speedo is off by 5 mph at ALL speeds (i.e. reads 5 when going 0, reads 55 when going 50, reads 155 when going 150, etc.) then pop the pointer off and put it back on where it points to 0 when going 0.

If on the other hand it's off by 10% (i.e. reads 11 when going 10, 44 when going 40, 88 when going 80, 154 when going 140, etc.) then it's time to look at speedo gears.

Figure out what your error format REALLY is before jumping to conclusions.