Originally Posted by briangp
You are dead on the nail, my friend. At least someone understands.
I'll beat this dead horse again... (I did base this off what I know and a few assumptions but believe them to be correct. If I'm incorrect, feel free to correct me)
From what I understand the ECU obtains data from the transmission in rotations of the output shaft (I don't think it comes from the wheel sensors, I think that's for ABS). So we all agree there is one point that the distance/speed is derived from. If you install larger tires, you will physically move faster than the output shaft indicates and if you install smaller tires you will be travelling slower than the output shaft indicates, assuming you don't change anything else like the rear end or reprogram the ECU.
So we have a single source which is now the ECU and it converts the pulses from the output shaft into a digital and analog representation. The digital is for the ODO and is highly accurate. Now the analog part is not so accurate. The ECU should create a voltage consistantly throughout the speedo range and is done with a digital to analog converter and that part would be consistant with known values. But the one variable that is not "Digital" and consistent would be the speedometer itself. It's magnetically driven and unless it's calibrated like a voltmeter, then it will not be as accurate. I'm sure there are acceptable tollerances for the needle like it's okay to say you are travelling faster than you physically are. We see that all the time.
Okay, I'm totally done with this dead horse and I think many of us are saying the same thing, just in a different way.