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Just started driving my 2007 F150 4.6 4x4 and have to say the economy stinks. It has less power and eats more fuel than my '77 F150 with the 400. On two different segments of long distance driving I got 14.68mpg and 14.75 mpg. This is unloaded driving across stretches of eastern Utah and I70 in Colorado. My '77 got a hair over 15 for the same trips. My '64 Dodge with the '86 318 engine got over 20! Now I do agree that power rack and pinion steering and A/C is nice, and it's nice to cut way down on wind noise. The headlights are better too. But the mileage is abysmal. 92,000 miles and it feels almost new, but I never expected to be less than 15 mpg!
Gearing could be an issue.... over 15 from a 400 is amazing and I am skeptical. Big fan of that underrated motor, but mileage is not the strong suite...
It's factory stock, no lift or leveling kit. By the axle codes I have a 3.55 (code #19) open differential.
RE: 400 getting good mileage - I was surprised too but the 3.00 rear and the 30.5" tall tires probably helped. I haven't driven it a lot so it's not a major claim I am making, also I had it jetted way lean for the mountains.