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The larger tires are much heavier, especially the thicker sidewalls of an M/T.
This weight must be turned , and as this weight increases, it consumes HP, and thus decreases MPG.

Decreases mpg, yes. But actual mpg is something other than what you'd figure out with displayed miles traveled and gallons consumed since the mileage is off due to larger diameter tires. So yes - the larger tire will be harder to turn and increase fuel consumption and decrease mileage, but the mpg figure is off due to the larger diameter. So what you'd figure out from the numbers, though it be lower than what it would be with stock tires, is incorrect. Shouldn't it actually equate to a higher number than what you'd figure from pump and odo numbers?