For the past year I have been consistently getting around 17 mpg mixed driving back and forth to the office in my 09 Lariat SCrew 4x4. I put on a set of 275/70/18 BFG All Terrain K/Os last week. They are approx. 1" taller than the stock 275/65/18 Wrangler SR-A's. I just filled up after running a full tank on the new tires. 12.54 MPG!!!!

This cannot possibly be simply a product of the tires can it? The truck is stock other than the new rubber. My driving this week has been very typical.

Any thoughts? Has anyone else seen the same thing?

Also, when they put them on, they put 35 psi in them. Where do you guys run your tires?

 

Same thing happenend here> I went from the stock 275's to a 35" tire and saw a dramatic drop. I don't think its as bad as it seems because with the larger tire you have less revolutions per mile.

 

Your speedo isn't accurate any more, so the odo reports less miles traveled.

 

http://www.miata.net/garage/tirecalc.html

275/65/18 is 629 Rev/Mile
275/70/18 is 608 Rev/Mile

So you have a difference of 1.0345% so your 12.54 mpg should actually be 12.97. So yah a little better then you think, but considering you have a heavy tire you're doing good

 

The SRA weighs 39 pounds. The TA KO weighs 57 pounds. That much rotational weight has a lot more impact than adding 100 pounds in your truck bed. I learned that with my 03 when I got the same tire, just 1" larger than stock and 3.5% taller gearing, similar to your change. I swapped the OEM tires before the first fillup, but never got more than 15mpg, corrected. They looked great, though! Well, this time I went back to stock size, found a relatively light weight 46 pound LT tire, the Revo 2. So far, I'm getting about 10% better fuel economy, about what I expect for my truck.

Also, check the tire pressure. Those tires are designed for a lot more pressure than P-rated per the driver's door sticker. I requested my tire dealer set them to 50psi and they set them to 35 per the door. I am getting much better FE at 50 now.

 

Run a few more tanks. Might just be driving habits. Fuel up after about 100 miles or so. That way your driving habits for the "short tank" are fresh in your mind. I just went through a bout of sporadic mpg droppage- ran 100 miles on the highway and topped back off. My frustrations were eased right away.