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Diameter
Yes, but 2% is nearly 2 mpg in the 60+mpg range you're talking about.
Here's an easy way to correct for odometer error. Drive ten miles by the mile markers on an Interstate and compare your beginning and ending odometer readings to to the ten miles. If your odometer lost 1/10th mile in ten miles (not uncommon), when you calculate you mileage multiply the miles driven by 1.01 and divide the answer by the gallons used for your tankful. If your odometer reads high for the ten mile test, subtract out the extra miles when calculating your mileage.
Another problem with measuring mileage, especially with very high mileage cars is getting consistent fill ups tank after tank. With modern cars with evaporative emissions systems you're not supposed to top up the tank. What I try to do locally is always fill my car at the same side of the same pump and use the auto shut off (no further topping up or rounding off). A tenth or two gallons of gas one way or the other can give big swings in mileage on your Prius.
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