ok....I know I'm running a lifted truck with 315 tires and 4:10 gears but doesnt 13mpg still sound really bad? 120 miles of the 200 i just drove was highway at 57 mph. the rest was in city and some extended idling for warm up before driving. also, my exhaust manifold is cracked...but I don't know how much this would affect it. What do ya think?

Lifted trucks are bad for mileage, especially with big tires. 4.10 would seem to help you a bit in this instance though. But you are still pushing big tires and having to push a lifted truck through the wind, the lower the truck the better. A cracked exhaust manifold would allow pressure to escape and in turn reduce the drive pressure that the turbo sees which means less boost. It would be inefficient. Do you have a chip on it that advances timing? That supposedly helps a lot. Same with an aftermarket lift pump. The VP44 advances timing hydraulically with fuel pressure. If your fuel pressure is low, you hardly have any hydraulic force to advance the timing, so it doesn't happen. This is why everyone sees a 3mpg or so increase when they go from a deadbeat stock pump, to an airdog or the like. Then there is the cold weather that drops it down as Mike has proven in the mpg thread.

ISX is right... Just dropping 20-25 pound per tire from 265/75 R16 to 235/85 R16 gain me MPG really quick. (Rolling resistance and rotational mass) Talk to THRacing his truck is 4.11 gear and getting some good MPG. Frontal area will always have a effect. The lower you can get the truck better the MPG's. When was the last time you saw a NASCAR standing 7 feet in the air? Not trying to be a punk but the fact is there is nothing efficent above standing tall in the air. Might look cool but that's it it look cool and your wallet is drained. Crack manifold is for sure dropping your turbo drive pressures... Lowering the boost... Black smoke (unburnt fuel) The extended idling will kill the MPG's too... I try to only idle till I see about 90-100*F on the ScanGauge II and then take off this allows the engine oil to at least be like summer start temps. It only takes me 2-3 minutes to this...

everything your saying makes perfect sense. It's just that I know people who have the same truck basically (big tires and lift) that claim much better fuel mileage than I am getting. i know what your saying is true however.I do have a airdog raptor 100 lift pump,fresh fuel filter,oil change,fresh K&N air filter. I am not seeing any burnt fuel out of the exhaust at all. I dont have a tuner yet either.It's hard telling when the valves have been adjusted if ever. adjusting the valves and replacing the manifold is my next step I guess.

this is about the same mileage that i get out of my truck with mostly in town driving. i get about 16 or so out on the highway at about 65-70 mph.

ok....I know I'm running a lifted truck with 315 tires and 4:10 gears but doesnt 13mpg still sound really bad? 120 miles of the 200 i just drove was highway at 57 mph. the rest was in city and some extended idling for warm up before driving. also, my exhaust manifold is cracked...but I don't know how much this would affect it. What do ya think?