wthalliii

I have a 2007 GT with 255/50ZR17 Nitto Invo's on the stock 17's. The Nitto web site lists the revolutions per mile for these tires as 768. Now, my tuner will only take increments of 5 for the revolutions per mile setting, so I set it to 770. But today, I was driving past one of those little radar trailers that the police put out to tell you your speed, and the radar said that I was doing 46 MPH, but my speedometer read 50 MPH. Now, I know that speedometers typically read a little higher than your actual speed, and I'm okay with that, but 46 vs. 50 is only 92% accuracy. That's not very good. What's going on here?

Okay, well, but regardless of the problem, if I increase the revolutions per mile setting by 8%, then by that calculation, the revolutions per mile should be about 830 instead of 770. Right? Or have I got that backwards?

Help me out here, please.

MrLatte

If it was me, I'd leave it set at 770. That was you're entering into the computer very close to the actual data for the tires. Otherwise you're just chasing two different speedometer readings with questionable calibration status.

parchisi

Do you have a GPS, or can borrow one? I would use one of those to calibrate my speedometer to, they're usually dead on.

wthalliii

Yes, I have a GPS. That's a good idea.