Speedo Brain teaser

I have a ScanGauge II, and was most concerned with it having as little odometer error as possible (can live with speedometer being off) in order to get accurate miles per gallon. It needs to be re-calibrated to adjust for tire wear, say every 10-15K miles. Same thing happens to vehicle odometer and speedometer, but cannot be adjusted. Have no idea what manufacturers use for accuracy, ie brand new tires, half-worn tires, etc. Used the mile marker comparison (do at least 50 miles). Going from stock tires to new (same size, diameter, and revs/mile) required about a 9% adjustment to get zero odometer error on ScanGauge. Truck odometer went from 1% under to 3% over with new tires. Truck speedometer is about 2 mph off. If you're serious about knowing your true mpg you must account for odometer error. From new to 3/32 tread remaining tire lost about an inch in diameter.