Originally Posted by
pizzaguy
Ok, but when I put the bigger tires on the back, my speedometer went from being 1mph off the GPS to agreeing with the GPS. BUT, at 80mph, I lost some ~350 RPM.
This implied to me that the sensors in the front wheels drive the speedometer, and others here agreed.
Oh, and my car is a six speed.
Assuming your feedback is valid on the loss of RPM (which I have no reason to doubt), then that verifies that the electronics do compensate for a change in rear wheel/tire setup. Bigger size would require lower RPM (drive shaft speed) to maintain equivalent gound speed. The fact that you've indicated a change in Speed readout indicates to me that the overall accuracy of the Speed is within an acceptable range, which would explain the small +/- change.
When you say the MPH changed, at what speed did your see that? Most Speedos aren't exactly linear throughout their operating range. Was always a problem back in "the good old days" of mechanical hardware. Most likely improved with today's electronics, but a smaller impact.
And per previously mentioned, there are quite a few sensors providing input to overall speed control of the XFire, which has to be a factor in what the Speedo reads out. I do recall seeing a different notation on the 6-speed manual. Gotta go back & look at it now!
Interesting thread!