2004 F-250 6.0 4x4 CC. Bought brand new. First highway trip, digital readout went up to 16.2 mpg (1000 mile trip). Put on a 4.5" Donahoe lift kit (real real smooth), and a set of 35" Toyo Open Countrys and mileage went down to 14.2. Went on another trip, took OFF 35" tires and put stock back on and digital readout NEVER changed (1000 mile trip). Came back home, put 35" tires back on, went to my parents (1250 mile trip) for Thanksgiving, and digital readout NEVER changed again. Hard to calculate the exact speed and odometer with larger tires but I figured putting the stock tires back on for the 1 trip, mileage should have improved. This really really sucks! I added a K&N air filter and that kind of sucks too! When you come off of speed fast, the intake makes a baffle loud noise. I called K&N and they said that was normal and it was too much air being sucked in and nowhere for it to go? so the air comes back out of the filter? I am not sure what the heck I have done now. Man, I went diesel for the MPG!!!! I have about 9,000 miles on the truck now.

 

It sounds as if you are getting better mileage than a gasser. Obviously, with a lift and larger tires, you're going to get worse mileage. You're in the 15 MPG range which seems typical for half of those here. I've got a 4x4 PSD CC Shortbed '03 still stock and I'm getting 17.5 MPG mixed and 22 HWY. What's done is done. I'd keep the lift and tires and not worry about the mileage.

 

You put a K&N filter on it. At what point in the set of events. Did you start getting lower milage after putting on the lift kit, big tires or K&N? Did you make sure the recharge oil on the filter was dry? Make sure only to put the oil on the fresh air side? Did it come with a new MAF sensor? Did you get recharge oil on the MAF?

As for the noise you get when backing off, all 6.0s will do that. It is nature of the beast because of the variable geometry turbos. There are many more advantages to the VGT.

 

Also are you resetting the overhead display when you fill up? The overhead display will continue to average the fuel consumption for quite a while - over several tankfulls anyway.

Reset it by pressing the two buttons at the same time. If you ever want to get an idea of your fuel consumption at that very moment, go ahead and reset the overhead. This will give you a quasi instantaneous reading.

From what I've heard the K&N flows more air, but also flows more dirt particles. This was measured by oil analysis and the silicon content in the oil.

The lift kit is going to put the truck up in the air more and won't be doing you any favors fuel mileage wise. The tires probably have higher rolling resistance than the stock tires as well.

Given that the stock tires are 31.65 inches and the new ones are 35, 35/31.65 = 1.106, and 1.106 x 14.5 = 16.035. If you haven't had the speedo recalibrated for the new tires, the truck is actually covering 1.106 times the distance that the trucks computer thinks it has. It is doing the MPG calculation with the old tires and that's probably part of the reason why you saw the initial mileage drop. Putting the stock tires back and resetting the overhead may get you back close to your factory stock MPG value.

If you hand calculate the mileage, fill the tank the same way each time - most folks prefer to fill to the filler neck. Multiply the miles traveled by 1.106 (assuming that your speedo wasn't recalibrated) and then divide that by the number of gallons used.

You should also consider spending $20 and having a Blackstone oil analysis done to check the air filter performance and make sure you aren't getting too much grit in through the K&N.

Hope this helps.

 

Change only one thing at a time and see what the effect of that change is before you change something else. That way you know the results of any given change.