I installed Nitto Terra Grapler 305/70/17 tires about 3000 miles ago, the differience from factory tire is great, the mileage change I haven't noticed any differience, the ride, and traction is 6 times better.

 

I've got a 6 spd mnl and my mileage dropped ditching the 265/70/17 and going with 285/70/17 I'm calling bs on the guys switching to the size I'm running or wider and claiming better milegae. I'm sorry but the physics of more mass and extra rolling resistance do not equal better fuel economy. A taller 265/75/17 would get better economy maybe even a 285/75/17 but any wider and I don't see it. With regards to Nitto tires I have seen more than a few complaints about them only lasting for 20K miles with normal driving.

 

No BS, actual measurements using GPS for speed and distance and for a 6 speed manual. From 265 highway tread BFGs to 315s M&S tread BFGs and 65 mph highway driving I gained an average of 2.3 mpg. Constant 70 mph I gained 2.1 mpg. The comparison was over about 9000 miles in both cases.

I agree that autos do not seem to get close to these gains. Autos have closer to an optimum final drive ratio for the smaller tires, I think.

I agree with the conventional wisdom that going to a larger diameter tire raises the vehicle and therefore more creates more air resistance and a wider tire has both more air resistance and rolling resistance. Agressive treads also roll with more resistence. All combine to make mileage worse. The 6 speed final drive ratio appears to not be optimum (for economy), however. The better economy rpm obtained by going to taller/bigger tires more than offsets all the increases in resistance.

 

What did your RPM's drop to ColdCase? Mine are around 2300 at 72 mph.

Something to ponder guys. This is just my opinion. I dont think these trucks are really suseptible to small changes in tires height or diamater as far as drag and wind resistance are concerned. I think the stock tire height is 31.6". If I go 33.6, I have only raised the vehicle 1", which unless I'm driving at NASCAR type speeds, is not going to affect the aerodynamics of my truck at all. Same thing with tire width. If I go from 265mm to 295 mm, I've only added about 4 3/4" of cantact patch. These trucks are designed to pull 10K-15K pounds. Almost 5" of extra contact patch, I believe, will not put any extra resistance on the motor. And if it does, it cant be enough to actually affect performance or mileage. BUT, the added tire height will reduce the engine speed at the same mph. Now you can get too tall and start getting the engine to run at too low of an RPM which can decrease mileage. Ever had to push a car? Did you have a certain speed at which the car was easier to maintain forward momentum? Starting off was tough, even moving slowly was not easy, but once you got her going at a nice pace it was pretty easy to push, eh? I think us guys with the 6spds benefit more from the taller tires due to the lack of a power robbing TC. This is all just my opinion. Please be nice while debunking my hypothesis.

 

It depends on your rear too. I have 4.10s and the truck revs high all the time above 60 mph so bigger tires will bring down my rpms giving it more the fuel mileage of a 3.73 mpg.

 

What he said...

 

A lot of this comes down to having the truck run in the "sweet" spot. Peak TQ and peak HP is where you want to be for the most part, there are times that fairly level ground will allow for that overdrive situation where the lower RPM's will yield better MPG but if a lugging situation occurs your mileage is going suffer, higher RPMs will get you the same situation as well you will then run up against the wind resistance, there is a point where wind resistance becomes a big factor. I just went back down to a 265 from a 285 for the towing, it puts my towing speed more into that "sweet" spot. I found that when I was in the sweet spot with the 285's I was running ~72 MPH, just a touch fast for me with 30" of camper behind me as well the wind resistance is brutal at that speed. This weeked I made a run to my Dad's empty, 227 miles one way, on the trip back I pulled his 14" car hauler with his Grizzly on it, I guess it at 2300lbs. Round trip I burned 3/4 tank of fuel, 454 miles @ 17.5MPG. I did that very same trip several months ago and pulled 12.8MPG. Thats running the speed limit or a touch over, the only change here is tire size. Sure dont look as good but boy she pulls better.

 

does rim width have an impact on tread touching or just side wall difference. I have 35x12.50 on a 18x9.5 rim. Would I benefit going to a rim size of 18x8?

 

The diameter difference between the BFG 265s and 315s is a little less than 10%. My rpms rpms dropped about 9.5% for a given MPH. 2000 rpm is just under 70 mph.

If you want to compare two tires, look at their diameters. The % difference in diameter will be the % rmp difference for a given mph, or the % mph difference for a given rpm. For example, if one tire is say 10% larger than another then the rpms would be 10% less for a given mph. A vehicle running 2300 rpm at 70, would run 2070 rpm (90% of 2300) at 70 with the larger tire.

As mentioned above, its all about the sweet spot. For the 06 tune unloaded, I think the spot is less than 2000 rpms. My best mileage ever by 3 mpg was on a back road 40 - 45 mph unloaded trip.

 

For anybody owning an '06 or newer, the largest size tire the dealer can reprogram into the computer is 285/70/R17 and it comes up as a BFG load rated D on the print out.

Lucky for me the programmed it into the CPU before my tires were installed, so I am going to install the 285/75/R17 Toyo AT's early next week...