Question for the math people

In an attempt to figure out my fuel economy with 33" tires, I've come up with an equation, but wanted to know if I'm close or way off. The numbers seem right, but I figured some of you math people could check my work.

[262.9 (miles shown) x 8.11 (% speedometer too slow)] + 262.9 = actual miles driven
so:
(262.9 x .0811) + 262.9 = 284.2

284.2m/15.004g = 18.94 mi per gallon

BTW, I used some info from this tire calculator based on comparisons of stock size (265/70/16) Vs. current size (33/10.5/15)

 

or you could get a gps. according to my gps which i believe is pretty damn accurate. for every 100 miles driven (gps) my odometer registers only about 90.5 miles

 

Unfortuanately I don't have a GPS, nor can I afford one. I must resort to good old fashion brain power. I'm sure there's a formula for this, I'm just not sure if I'm using the right numbers.