The easiest way to check the accuracy of your odometer/speedometer is to take a ride on a highway that has regular mile markers. Drive 10 miles per the mile markers, (mile markers are very accurately placed), taking note of the odometer setting at the start and the end. If your odometer says 10 miles you are dead on, if it says 9 you are reading 10% under actual, if it says 11 you are 10% over actual, 9.9 is 1% under actual. [NOTE: you do not need to hold any particular speed, both the odometer and speedometer will have the same percent of error].
Once you kinow that, then you can apply that to your existing calculation to get an accurate MPG reading. Also, you can then determine if you might want to regear or change the speedo gear. Speedo gears will adjust the reading by about 7% per tooth added or subtracted. So if you are reading under 14% you would want to remove 2 teeth to make it accurate. If the reading is off by a large percentage, you would probably have to regear the differential, which may not be a bad idea anyway if you only changed tire size and did nothing to the gearing. Usually, if you increase tire size without regearing you will seriously suffer on the power end.
Hope that helps.
Good luck,
>-
