[gone-ot]

...first thing I did when we got our new '09 Vibe was have the speedometer calibrated, and it was found to be "off" by a consistent 2mph LOW--actual 48mph, speedo 50mph--all the way from 20mph thru 80mph. But, since the speedometer is actually a stepper-motor driven from the computer, it seems like the computer is programmed to "add" 2mph to whatever input comes from the wheel rotation sensor.

...however, that raises a question about "what's happening?" with the odometer...is it being incremented from the "direct" signal (usually 1,000 pulses per mile) or is it ALSO being "tweeked" by the computer? Best I can tell using the Interstate mile markers is that it's responding to the "direct" signal, but short of getting a GPS unit, how would one check this with certainty?

...I ask because the Arizona Department of Transportation (ADOT) says the "mile" markers are NOT guaranteed to be exactly a mile apart, only approximately so, within the error limits of the calibrated "5th-wheel" that they use, and the fact that mile posts are placed equally on either side of the roadway, so lots of "one-sided" bends/curves in the road can slowly accumulate into a noticable error. I asked about divided highways, and was told they only measure via one side, but didn't say which.