shamblin123

Here's the story on your 35 inch tires and fuel mileage.
I just measured the rolling radius on my stock 265s at between 15 1/4 and 15 5/16 inches which translates to 8 feet per revolution or 660 revolutions per mile.

Your 35 inch tires will have a rolling radius of around 17 3/16 inches depending on inflation pressure which translates to 9 feet per revolution or 587 revolutions per mile. For each mile your odometer registers, you have actually traveled 1 1/8 miles - 8 miles on the odometer equals 9 miles traveled. That could account for a 12.5% fuel mileage discrepancy.

However, that is not the whole story. It takes more energy to overcome the rolling resistance of the wide footprint on the road and probably more energy to just turn the increased mass of your tires. Lower inflation pressures will increase the rolling resistance. You have also increased the frontal area for increased wind resistance - larger tires being forced through the air and a raised profile. The air doesn't flow as freely under the vehicle as around and over the body due to turbulence from all those projections underneath.

Wind resistance is no small issue. I had an E350 Van that got in the neighborhood of 16 MPG at 45 MPH and dropped to about 10 MPG at 75 MPH.

Many delivery trucks use skinny high-pressure tires to overcome rolling resistance.

Energy used equals fuel burned!

F250 6.0L 5 speed auto XLT extended cab, Short Box 4X4. 18 - 19 MPG empty mountain highway miles over 10,000 - 12,000 foot passes and 12 - 13 MPG towing a 6000 pound trailer over two 10,000 foot passes.