My DIC always reads around 12.2-12.5 mpg's and when I check it by man calc its right, I used to get around 14 city and 17-19 hwy at 80 I would get 16.5 but now I get maybe 15-16 hwy and at 80 about 14.5. I did change tires from 305-50-20xl's to 285-55-20 michelin load e's, I did recal the speedo, I run a borla 4" exh, hypertech on level 3, air filter has maybe 5k on it same with fuel filter.

Maybe its a coincedence but a couple months ago I was in Parker AZ, towing my 30' WW home and stopped for fuel, I swear as I was filling up I was smelling a slight gasoline smell from the diesel fuel, I ran in and had the guy come out and check and he said its just the ULSD and maybe they put it in the LSD tank. I was skeptical but figured I'd know right away if there was gas mixed with the diesel, seemed OK but it also seemed thats when my economy dropped but thats about 2 months after I put on the tires as well.

I would think at HWY speeds and with 80psi in the tires even though they are slightly taller mileage shouldnt suffer much.

I have noticed a power drop towing compared to the slighly shorter and lighter tires, could a gear change help?

Just drop down to 265's and you will do a lot better towing. I tow a pretty heavy 5th with mine, and I noticed almost no difference between the 245's and the 265's towing.

I've seen about a 2 mpg drop going to 265's from 245's but when trailering it is far more stable and rides better with the 265's so I'll live with the drop in mileage.

Has your cooling fan been running lately, that will hurt your MPG.

make of tires has alot to do with mpg's.. I went from the 215/85's, to 245/75's (less then 0.28% difference), but the wider tread, and more aggressive vs street tread cost me between 2 and 2.5 mpg's..