I think you folks need to get your Speedo error figured out. Some of you are saying horibly low MPG. well oversized tires is going to make your odometer run slow. Your probably getting a lot better then you think.

If you have a tach you can figure out you actual speed using these calculators and the nice gearing charts posted above.
http://www.ring-pinion.com/calculators/calculatorsind.html

I use a GPS to check both speedometer and odometer. I've often found that the error is different on each. Put in the GPS and drive for at least 10 miles and compare the GPS reading to the odometer reading to determine the error. If its a small error (less than 10%) you may need to drive a longer distance to get an accurate measurement, since its hard to resolve to anything less than 1/10 mi on the odometer.

Bob