[OutthaholeFX2]

I lost 4mpg when I changed from 17's to 20's on my Cummins that I had. They were a lil taller and wider for more rotating mass.

 

[OutthaholeFX2]

I went from 17's to 20's on my '05. Overall tire height and width increased quite a bit too. Initially, mpg dropped 2-4mpg. Then I had to PCM re flashed at my local Ford dealer and the difference was negligible. Size is not really the issue as much as how your PCM is set, i.e. correct odometer readings, and weight and how much tire is contacting the road. The tread pattern can have an effect as well.

I had the best programmer money could buy on my truck for the Cummins(Smarty S-06)
and had my taller tires matched perfectly with the odometer and did hand caculated mpg readings and still lost 4mpg due to the larger "Size". The larger size increased the weight by over 30lbs each compared to my stock 17's. Plus the xtra width making more contact with the road but they also stuck out over 1" past the fenderwells catching more wind drag too. So i would have to respectfully disagree and say that size is the main issue. I also agree that it is very important to have the odometer set to match your tire height vs. gear ratio to get acccurate mpg calculations at the pump. Just not the determining factor to poor fuel mileage after going to a larger tire/wheel combo. Larger size=More weight+Rolling mass=Motor works harder to move them=use more boost=burn more fuel=worse fuel mileage.

 

[OutthaholeFX2]

There is also something to be said for increasing the rotational inertia, the more you increase the weight the farther from the center of the axle shaft, the greater the impact.

Ex: If you try to spin a weight at the end of a 1' rope vs the same weight at the end of a 2' rope. The 2 ft rope takes more effort to spin & to stop.

This is also true which reminds me of more bad news. Along with less fuel mileage you will use more brake to stop causing less brake life but I think its worth it if when you walk away from your truck and after 20yrds away you turn around to look again before walking off. If the mods don't do that for me,I would'nt do it and save a LOT of money! Lol