I think it'll hurt your mileage IMO... more rotational mass to get moving, less gearing to help get it done. I lost about 3~3.5 maybe 4 mpg when I went to my 33's. they weigh in at 76 lbs each new... 20~25 lbs heavier than the stockers.

 

Steve, I was expecting a drop in milage when I switched for the exact reasons that you mention. But I saw no change. I attribute that to running a few hundred lower rpm at highway speed which is 90% of my driving. Doing more stop and go driving or even more of an even mix of highway/city I think you would be right on the money.

 

Welcome aboard Willie. It is good to see a fellow SC redneck on the forum. Your mpgs should not change much if you are doing hwy driving.

 

IDK... when I 1st got my truck I managed 18.5 IIRC going down to Palm Springs with stock tires doing the speed limit the whole way... never saw that again until about 6~7 months ago. And believe me, I definately have tried to get the best mileage out of it on trips... I think between the width & weight of my perticular tire, it is at it's worst... I dont remember how wide the stocker was, 265?? but the current tire is 325, take the delta and double it to account for both sides. more surface area for sure contributed to my drop. going from 265 to 285 may not make a difference.

So when the next guy mounts his 285's maybe he can do some mileage comparison testing for us here... just make sure you account for the odometer difference. (8.5% on my 2000 QC 4x4...)

 

Well change in tire size may make a difference, but not always necessarily due to the size. Because when you change the size of the tire without recalibrating the oddometer for the size, your "percieved numbers" will be different.
Also take into account that a taller tire effectively reduces your gear ratio. thus turning less RPMs for the same speed (again assuming your speedometer is corrected after the tire size change).
for example going from 265-70/17 to 285-70/17 makes about a 3 percent difference in gear ratio (assuming you have 3.73:1 gears it lowers it to 3.62).
Changing to 33" tires changes that ratio by about 4.3% (from 3.73 to 3.57).
As for rolling resistance well it would take a HUGE difference to make a HUGE difference, I belive an increase of 20 percent in rolling resistance equates to something like 3-5 percent.
even if each tire weighed 50 pounds more each I believe the entire weight of the vehicle is used in the calculation as far as the force used to calculate the rolling resistance, so I think the increase would be kind of minimal.

So, is there really a change in fuel mileage...maybe a percieved change, but I am not convinced the change in tire size causes a decrease in mileage. if anything the effective gear ratio change would compensate for any change caused by rolling resistance.

I guess one would need to make distance measurements by GPS to prove or disprove this.

 

Well, I FINALLY got my new wheel today and i dropped off my wheels and tires to have them mounted. I'll let you know how I make out tomorrow.

For those of you who want to compare your mileage, check out this site

HERE