[MetroMPG]

I second (third? fourth?) the advice to invest in a ScanGauge. If finances are tight, you could always re-sell it in a few months and get most of your money back (after using it to tweak your technique).

As for the wheels: if the tire diameter of the ones your dad added is larger than stock, your odometer could be under-reporting your distance travelled. The result of that is your MPG calcs would show that you're getting worse mileage than you actually are. Just a thought.