I'm currently at a loss as to what could be causing my speedometer to read around 20 to 30% high when I'm driving at 60mph. I have a 1986 base RX7 and I've looked into the usual suspects.

The tire size is within 0.4% of stock size, and the speedometer read even faster before I made the switch to my current wheels (the tires that came on the car were smaller than stock size).

I looked into the final gear ratio, and it appears to be 4.1, same as stock.

I took a look at the speedometer driven gear in the transmission to see if it was a different size, but it's the same 20 tooth gear that came stock on the car.

I'm running out of ideas as to what I should look at next to diagnose this issue. Any help would be greatly appreciated!

 

At this point, I would I would swap in another speedometer from another car and see if the issue remains. The gears in the speedometer can wear and cause issues with accuracy. Have you inspected the speedometer cable as well?

 

About 5 years ago, the speedo in my S5 started reading 2x then 3x times my actual speed. I just replaced the entire instrument cluster with a used one. I think there were some electronics on the speedo PCB that were going bad.

 

I've looked at both the speedometer cable and the instrument cluster themselves and nothing appears out of place/broken. Regardless, I'll try replacing both and see what sort of impact they have on the speedometer readings.