clemson725

I just got back from an interstate trip on I-20. My truck is an all stock '06 2500with new 285/70/17 E BFG all terrians, 43,000 miles on the odo., 48RE trans, 3.73 ratio. I aired the fronts to 60psi and the backs at 55psi. The truck was unloaded except for my clothes and dog. As as side note, after 900 miles my tires still had the little flaps of rubber on them from the mold....even in the center of the tire. The air temp ranged from 70-85 degrees with little to no wind and light traffic. I never hit over 72mph and only stopped 3 or 4 times. The best mileage that I got was just a shade over 20mpg and the worst was 19 on the money. I used no additives to the fuel. My gps had my average moving speed at 64mph. I calculated the mileage two ways. I did an adjustment to the truck's odometer readout and I checked it by the mileage my gps said that I had gone. Both were equal.

For the way I drove, I can't see anyone getting over possibly 21mpg out of a set up like mine. I will say that I've gotten more than 20mpg driving on 55 and 65mph highways, but going 70mph.......I don't see it happening.

BroncoAZ

My best trips on stock tires:

19.36 MPG cruising at 70 mph on country highways (some hills), overhead read 21.8
19.47 MPG running around town on freeways (65 mph), overhead read 17.8
21.31 MPG running 75 mph on interstate 10 (totally flat), overhead read 18.6

These single tank averages could be largely effected by inconsistant topoffs. I haven't done a long trip where I could calc a consistant 3-4 tank average. I'm looking to go to a larger tire to decrease my highway RPM.

clemson725

I went to a larger tire mainly for looks. I feel like a stock tire size would have probably netted a better mpg number. Those E load range tires are really heavy. the rotating mass is noticable. A lot of guys claim that they cant tell a difference between 285s and 265s, and truthfully, neither can i since i've gotten used to the 285s. BUT when i first put them on, oh yeah, big difference. Its there. You can't deny physics. You esentially increase the rolling resistance due to rotational mass and at the same time decrease your gear ratio.

I should have came back on I-10 to see what I could have gotten on it.

BroncoAZ

According to the specs on tirerack.com our factory BFG rugged trail tires weigh 51 lb, and your 285 BFG's weigh 59 lb. 32lb doesn't sound like too much difference on a 7K truck, but I realize it's rotating mass.

clemson725

Yes, rotational mass is a different monster than static weight. I wish i could remember how to do it from back in school, but in one of my classes we did a calculation on how tire size effects vehicle performance. I can't remember the tire sizes we used, but I do remember that we change to a tire a couple of sizes bigger and the car itsself saw a force of a tire that was twice the diameter of the original due to rotational mass. That pretty much means taht if you could theoretically keep the rotational mass the same, you could put 62 inch tires on the truck and you would see the same resistance as you do with a couple of sizes bigger. Or something like that. Its been a while since I was in school...

What is even more interesting is seeing the numbers crunched to see how much the force due to wind resistance climbs for every increment over 55mph. Its drastic.

yellowhemi

I just got back from a trip and average for 2 days was 20.9, mine is also 48RE with 3.73's.

clemson725

Pop your tire size up there yellowhemi.