And again, I have documented mileages to show this is not the case on the highway...in the city, it probably does cost mileage because you have to get that mass rolling; but on the highway I saw a consistent increase in mileage at the same speed (speed based on a GPS, not by the speedometer)...and that was running 35" BFG AT/KOs.

One thing to remember is that rolling resistance has more to do with tire construction than tire weight. I can show a documented decrease in mileage by about 1.0mpg just by changing tires, but staying the same size (changing from a 265 Pirelli STRa to a 265 Bridgestone Revo II). Its not just the mass. If it was simply mass, then your mileage would change everytime your fueled up, added a passenger, etc...there's not THAT much difference in weight between a normal sized tire (around a 31/32) and a 33/35.

BTW, I log every tank of fuel...I have 211k miles of logs for this truck alone.

 

That is exactly why I was surprised losing MPG in my scenario. My tire/wheel combo went from a 285/75-16 BFG AT/KO on stock rims to a 285/75-17 Nitto Dura on Alcoa forged aluminum wheels. The new tire/wheel combo was only 5lbs heavier than the one it replaced and they were a highway tread tire of the same width. It should have equated to lower rolling resistance, same width (drag) and slightly higher overall gear to equal better mileage. But it didn't, at least not overall under mixed driving conditions.

Lost of factors to consider.

 

I agree completely. I'm sure the width of my tires has more effect on mileage than the weight does. Even tread pattern can make a big difference. The tires previous to these were the same size and make, but Mud Terrains and they cost me near 4 mpg over the stockers!

Just out of curiosity though, how are you figuring your fuel mileage when your 35" tires throw off not only the speedometer (which you state you use the GPS for) but the odometer as well? I'm assuming your PCM been not been flashed to compensate for the taller tires since your are using your GPS to monitor your speed.

 

I used the trip in the GPS, as well as the odometer, just to verify my findings...plus I knew how far it was from point to point (a few miles off over a 400 mile trip means little). The mileage read by the odometer is actually LESS than that physically traveled (by a lot actually). And even using the odometer mileage, I was getting over 20mpg hand calculated.

Think about it this way...if normally you travel 200 miles with normal tires, larger tires are going to register less mileage (say 180) without recalibrating. You use X gallons to travel the distance, say 10 gallons. The actual mileage of 200 miles/10 gallons is 20mpg whereas the uncorrected mileage is showing 18mpg...going from the odometer actually shorts your fuel mileage!

I want to remember I was showing 0.9 miles while traveling about 1.0 miles, based on the roadside mile markers. That means for every 10 miles you actually only show 9 miles on the odometer...in 100 miles you've clocked 90 miles...see where I'm going?

 

Lots of pretty good posts here! Just for the hell of it...(most of the time) for approximately every 60 miles i drive, i have really driven around 6 more miles than it actually reads on the odo. I have 265/70/19.5s on my 04. These little things are good for those of us that rack the miles up on our trucks

On another note...going to try out a Smarty one of these days and see if that can help the mpg's a little

 

I have the Smarty Jr, and while I can't say it increased my mileage (more than I was able to achieved without it); it has made it easier to obtain those mileages more consistently.