Speedometer calibration (88.5 US Testarossa)

What matters is the effective OD (rolling radius) of the tire, not the wheel diameter size. You must be running a huge tire as it's more typical for manufacturers to have their speedometers read a little high (so the actual speed is a little lower or, worst case, accurate) -- or the calibration was just never correct. What rear tire size are you using?

This thread has some information from Adrian about how to calibrate a 308 speedometer that uses the same speedometer sensor as your TR:

https://www.ferrarichat.com/forum/threads/1982-308-gtsi-speedometer-wrong.455815/#post-143325739

but since you have your own calibration standard (GPS) you could just try tweaking the potentiometer on your speedometer until you get what you want.

 

That rear tire size is totally reasonable, and should cause less than a 1% increase in actual speed so your issue has to be on the electrical side of things. Good Hunting!

 

I checked on line and the 1988 should have originally come with 255/50R16 in back

P255/50R16 --> 255mm x .50 x 2 + 16" rims = 26.03" OD
P295/35R18 --> 295mm x .35 x 2 + 18" rims = 26.13" OD

The outside diameters are so close, I wouldn't think it would negatively affect the speedometer to this degree. Perhaps there is a failure in the electronics (as I think Steve was indicating above there).

Ray