Well first your average will be off. With the bigger tires, your driving further than the odometer reads. So when you average it your off. Uncorrected Im 2 to 3 MPG off. For every mile I actually drive, my odometer reads.8 miles.

I pull 12 MPG and theats with big tires and 5:13 gears. Yo should see at 10 MPG but should really see 12 13 on the highway.

Id start with a tune up and maybe new o2 sensor.
Exhaust work made some pretty good improvements for me

 

I get 8.8 in town, which is driving around my jobsite and starting the truck no less than ten times and on the highway around 11-12. Best ever was 14 bed fully loaded and pulling the boat to lake powell!

 

When I bought the truck I was told it was just tuned up. I have the papers but I didn't see the mileage on it. I am going to hook my truck up to the scanner next time I take it to work and see if the 02 sensors are switching fast enough. They cost $70 for 1, glad I only have 2.
Would an intake system increase my MPG? I am going to put an exhaust system on it soon. The type I talked to you (V10Sport) about a few weeks ago, still looking for some good headers for the right $$.
Since I'll probably be doing a tune up on it soon is there any performance ignition parts out there for the v10? Best plugs, wires, any thing else? I used Bosch platinum +4 on my car that was a mistate.
Thanks.