The 285's that are on your truck will account for a 7.7% (based off 245's stock) diffence in gas milage mathimatically, based on standard tire size conversions. With that being said, I get an average MPG of 11.9 before adjustments for my tires and 12.8 MPG after. My light comes on at about 240-250 on average, but that never equates to 1/8th of a tank...I'm still usually about 1 gallon about 1/8th.

 

make sure your tires are properly inflated.

"Every 10 psi of underinflation represents approximately 1% penalty in fuel economy." - the physics of mpg

 

I agree with some of these guys. Don't really go by the guage. Mine sits on right on E and when I go to fill it up it will only for like 22 gallons or so. The best way to tell is to fill it up, reset the trip and drive it for a while. Then go fill up again and look at how much actually went into the tank, don't even need to look at the guage really.

I was getting really bad milage too (10 city and 13 highway) but after some work I've been getting great lately. Like 12-13 in town and on my way here to school (330 mile trip) I got 17.25 mpg. Heres what I did over like a month period and noticed a gain.

1. Ran some injector cleaner through it
2. Ran a can of Mopar Combustion chamber cleaner through it
3. Ran some Marvel Mystery Oil into the gas tank (the recomended amount of the bottle)
4. Changed my oil and used the "Q Power 4x4 & SUV"
5. Same time as oil I put 1/2 a quart of the Marvel Mystery Oil into the crank, and its still in there.
6. Replaced my crapy Fram air filter with a K&N Drop in. The Fram wasn't even a year old either.

 

this is all great info.thanks guys.
ill check/clean the 02's.i always keep the tires properly inflated and will try the ideas that socha stated.
thanks again
happy halloween