Mpg help!

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Jesus you're one unlikable motherfucker.

 

1 mile = 5,280 feet = 63,360 inches.
A tire, 27 inches in diameter, will have to rotate a little over 747 times to cover that distance.
A tire, 33 inches in diameter, will have to rotate a little over 611 times to cover that distance.

Given that ratio, the 33 inch tire covers 1 mile using only about .82 times the number of revolutions. In other words, for a given total distance, your odo thinks you only traveled .82 times the actual distance. Using 375 miles with the original tires, with the new tires and uncompensated odo, the odo will think you only covered 307.5 miles even though you actually traveled 375.

Then you add the compounding factors such as added inertial weight from heavier wheels/tires, higher engine rpms at any speed indicated by odo (since the odo will be reading a lower speed than actual), etc.

Using the same figures/analogy, if you were getting 375 odo miles per tank before the tire change, and there was NOTHING else that affected your mpg, you would only see 307.5 miles per tank after the change even though you ACTUALLY traveled 375 miles with that tank.