Normally, in my day to day driving (90% city) my 2004 XCab 4.6L gets around 14mpg.

I took a trip to WV last week and got 20mpg on the highway doing 75+ the whole way (15% city driving per tank). It was consistent through three tanks of gas too.

I know highway driving on cruise gets you better gas mileage, but is 6mpg normal?

Could it be a difference in gas types, additives?

 

That is normal.

High geared. Computer controlled. Very heavy. Cold vs. warm.

Get the truck up to cruise speed and it costs very little to cover miles. The high gears, heavy weight costs a bundle to push up from a stop sign while the overdrive gear with the locked converter equals very little engine RPM on the freeway.

EPA is 15-19. This was with the engine warm. If you test your warm mileage against your cold mileage you will find you get 13 for the first five miles trip, 16 for the 5 mile trip back. To get the best mileage drive 55 on flatland with the cruise. 24 is possible.
Chris

 

It sounds a little high.....the starting point of 14-15mpg is normal but an additional 6mpg highway seems high. 17-18mpg I would think is the high end but if that's what you calculated your mpg too be....then that's what it is.

 

Well 6mpg from cruise is not normal at all. But going from 14 mpg with 90% city driving to 20 mpg 85% highway driving is very normal. I go from 15 city, to about 19 highway all the time, not even caring about fuel mileage. I know the 5.4's are rated at 14 city and 18 highway I believe. So your truck should get 20+, good for you man.