Originally Posted by brandonjansen
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).
In my typing up of my response I was thinking corrected speedo.. You are absolutely correct though, uncorrected speedo + hand calculated will result in wrong result.
Thanks for catching that..