Originally Posted by NWRQC
Because this computer thinks that he is driving less distance than he really is. His hand calculated milage probably didn't change much, but the unreliable computer calculated fuel mileage is going to be off.
His hand calculated mileage will be just as far off because his odometer is showing a much shorter distance than what he is actually driving. Therefore he is using even more fuel than stock (because of the larger heavier tires) but going a much shorter distance (because his odometer is wrong).