[doviatt]

On your trip....note the odometer reading coincidence with a mile marker. Then (the more miles the better) see what it reads after 10 miles or so.
Example: smaller tire or high odo reading.
Start: End:
ODO: 252.3 ODO:262.5 = 10.2 miles
Mile marker 10 Mile marker 20 = 10.0 miles

10.2 odo miles divided by 10 actual miles = 1.02 Take the inverse of this 1/your number 1.02 = Calibration number is .980
Every time you fill up multiply your odometer reading by .980 to get your actual miles. (example 157.8 miles X .980 calibration = 154.64 actual miles.)

Edit: oops you could have also divided the 10.0 by 10.2 for the same result. Sorry. Not thinking straight.

If your odometer is off the other way or your tire is larger your odo number will read lower numbers and the calibration number will need to be greater than 1.0. My calibration number ends up being 1.0595.

Also verify this with a GPS and or Google maps. Your route to work would be a good one to use as it is the same every day. The GPS, the google map route, the MPGuino, and the odometer with the calibration should all now agree.