REELNAUTI4U

I think what you will find is alot of people rely on their factory gauge to calculate fuel economy. These devices are refered too as "lie o meters". Aftermarket tires can also fudge these figures whether to benefit their results or not.
Diesel truck owners that claim anything over 22MPG for speeds at or in excess of 70MPH need to hand calculate mileage for real world results.

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I Fish 2

It seems almost everyone in this discussion is overlooking one of the most important parts, the rear end gear ratio's. If they are low, like 4.11, you are not going to get the same mileage as someone who has high gears, like 3.73. The 3.73 will see a bigger drop in mpg's when towing a load, vs. the 4.11, but if you are not towing much, or often, they are the way to go. My uncle and I had identical trucks ('96 Ford F-250, 4x4 extended cab, diesels), except his had 4.11 gears and he averaged about 13mpg's empty and 12mpg's loaded or towing a horse trailer. My truck had 3.73's and I averaged 18.5 mpg's empty, and, pulling the same trailer, I got 14mpg's in the flat land, and 12.5 to 13 in the hills. MPG's were all hand calculated, using our fuel logs.