Nuke

The "sweet spot" for the 4.6L is about 1600-1800 RPM. Anything over that and you're fuel mileage drops off drastically. I'm sure the 5.0 has a similar if not the same sweet spot. But who the f**k can drive at that engine speed on the open road? At 1800 RPM in 5th gear with my 4.6L, 3.55 rear and 275/40/18's I'm running roughly 60 MPH. I had to do a full tank on a few occassions at 60 MPH in Ohio and even had to do more than 2 tanks at 55 MPH on the Natchez Trace; both instances I posted 27 + MPG. For those too-many-to-count times I ran Chicago-to-Kentucky (400 miles all highway one way) every 2 weeks, at 70 MPH steady state I averaged 24-25 MPG. Throw in some short sprints to 120-130 MPH and the average dropped by 1-2 MPG. And when I filled the tank with ethanol laced fuel vs pure gas, I often lost 3-4 MPG. There are lot of variables but driver habits are probably 75-90% of the reason for not getting the advertised mileage.

Falcon Fixer

2011 GT with full exhaust, intake and tune with 3.31 gears =

23 mpg Average

27-29 mpg at 60

26.7 mpg over 500 mile trip at 70

Nuke

I hope everyone has been hand calculating their fuel mileage to see how accurate (or inaccurate as is the case with the vast majority of the 05's thru 10's) the onboard is. Typically, the onboards on the 4.6L and 4.0L reads 1.5-2 MPG high. Although my wife's '06 4.0L is about dead-on as are a few others. My '08 Toyota SUV is worse. It's 2.0-2.5 MPG off on aveagae.

Zodiac

I've been keeping track of my fuel economy with fuelly and I'm averaging 18.8mpg. My best so far has been 21.1mpg highway at around 75mph.

My daily trip is about 3 miles suburban roads and 30 miles highway round trip. I do 70mph in the morning on the way to work(at 4am in the morning I play it a little more cautious) and about 75-80mph on the way home.

My car's avg mpg is reading 19mpg, so it's not off by much at all.

fujimo

my 2012 auto with steeda custom 93 tune gets 24 on long trips with cruise set at 75.and I check the fuel milage the right way. fill her up and set the trip.

Mach2

Yep, I reset the trip mileage and the fuel mpg average screen to keep up with the mpg.

808muscle

I get a 150 miles per tankful! LOL

Gary Ugarek

If I get 20 I feel lucky, city mileage is about 10-12 HWY 14-17

Mach2

Well fine. I'm not gonna gripe then with the 13-14 city and 22-23 freeways. Year and a half ago I had a 2010 C6 Vette modded with 400 rwhp LS3, 357 cu in, that got 14 city & 26-27 freeway though, that's why I'm asking. Just in case you're thinkin it, NO I'm not gonna get a 2010 C6 then..........

White05GT

Fords mpg figures are probably based on 100% gas too instead of the ethanol laced gas we're forced to use in the real world. That alone will drop mileage 2-3 mpg.