I have a 2004 F-250 4x4 CC with 6.0 diesel. When I first bought the truck, I made a road trip (about 1000 miles), and digital readout said at best 16.2mpg. I then added a 4.5" Donahoe lift kit, and a set of Toyo 35" Open Countries, digital readout says 14.2 (that is in town driving now with the larger tires and lift kit). Went on another road trip (about 1000 miles) took OFF the 35" tires and mileage on digital readout NEVER changed. Went to my parents this Thanksgiving (again aboug 1000 miles), and digital readout says 14.1 now. I know the lift kit changed the aerodynamics of the truck but I would have thought with the stock tires back on, and 9,000 miles now on the truck, it should have improved? I added a K&N air intake. Easy to install but I have to get with K&N now because it makes a baffle type noise when you come off the gas hard. They said it was too much air and it could not get through the turbo and was coming back out the air filter?

Anyway, curious to see what you may be experiencing:
-will the Superlift odometer kit for 100.00 solve my problem?.
-how about the magnaflow 4" exhaust from turbo back for 650.00 installed (lifetime warranty!!!).
p.s. when my wife drives at 80mph (shows about 73 or so on speedometer when running larger tires on the interstate, fuel mileage sucks!!!!)

I own a 2003 F-350 6.0 bone stock makes 19mpg best 15 lowest when towing or playing. Your digital readout isnt always right try to do the math yourself you might get more.

When you change the rim/tire combo from stock to a larger r/t combo the truck should be going faster than the speedo indicates ex. speedo says 55 mph actual is- 61 mph. And no, the mods will only suck down more gas.
more air into engine = more power = more gas = low fuel milage.

more air and gas thing is right, but if you dont hotdog around, there shouldn't be much differances.

Also I think you sould keep things stock, you can get more power but only with major changes.

When you change tire size, you change the number of revolutions per mile the axle has to turn. The PCM uses revs/mile to calculate speed to send to the HEC module with in turn is used by the OTC (overhead trip computer) to calculate fuel economy. You must also reset the OTC fuel economy regularly or it will get so much data to average that it will no give you an accurate milage. Ford SMT

You can have the computer reprogramed for the tire size change :)

yes you can reprogram the computer to change the tire size. also, the air rushing noise that you are probably hearing coming out of a hard accel is the new vgt turbo. if you still have doubts, go test drive a new 6.0L.

also, since your odomitter is off due to the wheel size change, that will throw off your milage computer. finnally, lifting you truck makes it bigger, which makes it have to move more air out of it's way for it to continue traveling down the highway, which means it takes more power, and uses more diesel to make that power. face it, you are going to lose gas milage when you lift your truck.