I thought I would give you guys my mileage results on my recent trip from North Carolina to Alabama and back. My truck is a 4.6 xlt scab 2wd with 3.55 open gears. On the trip down I used a tank at 70 mph and 2000 rpm and got 19.5 mpg. On the way back I used a tank at 77 mph and the first mark above 2000 rpm (2100 or 2200?) and my mileage dropped to 14.9 mpg. It was a seven hour drive so the extra speed made it seem like a 49 miles shorter trip on the way back but my gas mileage took a big dive. These trucks seem to favor the 2000 rpm or less range.

 

You didn't mention anything about wind, hills, etc. so I assume it was all flat with low wind speeds...

 

Wind was virtually nil the entire trip and the landscape had no major hills or factors that would affect the MPG outcome. I went down I-95 from Fayetteville, NC to Florence, SC and then took I-20 all the way from Florence to exit 5 on the western Georgia stateline. The greastest difference in elevation was probably only a couple hundred yards during the whole trip with a very gradual incline between any high or low areas. The truck truly seemed to like 2000 rpm so much more than the 2100 to 2200 range. The engine even sounded much more worked when it was over 2100 rpm.

 

how many miles does your truck have? and what kind of mileage did you get when you first bought it?

 

It had 2300 miles when I left and it has 3700 now. I have been getting consistant 15-16 mpg city driving around town so the first tank was what I expected but the second was kind of a shock. I refueled to about the exact same point each time I refuel (wait until the pump shuts off and squeeze in an extra half gallon). I also didn't refuel on an unlevel surface so it had to be the speed that killed the mileage. By the way the tank on the trip down was the 8th tank and the tank on the way back was the 10th. I used tank number 9 around town while on vacation. I document everything in a small book. I got better mileage on tank 9 than I did on tank 10.

 

Could be a multitude of factors contributing to this.
Here is what I am thinking....
There is a gradual slope of about 200 feet down towards Alabama (this is based on the mean elevation of the two states), What you experienced may be greater or lesser depending on where you went.

The temperature of the air in which you are burning the fuel affects gas milage. Extreme temps one way or another will impact gas milage (I don't know what the temps are around there when you were there).

Different gas companies and stations within these companies carry different fuel. In the summer, I can buy gasoline with NO ethanol around my home area, which is very common. A trip of 75 miles south finds gas that is blended with 10% ethanol as part of the Clean Air Act. You may have purchased gas with a diffrent chemical make up each time.

My own truck seemed to get progressivly less miles per gallon until it reached 10,000 miles then leveled off.

Lastly, and most obviously, you had your engine spinning at 200 revs per minute on the way back up. This translates to 50 more "power strokes" per minute than the first time. This of course is going to burn more fuel.

I find that you are much more accurate in comparing gas milage when you average several readings. Now on your next trip, cruise at 2200 on the way down, and 2000 on the way back. Fill up the same way, at the same stations if possibe to eliminate other sources or error. Average the same RPM gas milage numbers together and see what you get. I imagine it will be pretty darn close to eachother, with the slower engine rpm slightly higher.
This sounds like a fun project, let me know how it goes. I would be interested to see what you come up with, if you try it out. I hope my crazy ideas haven't got you cross eyed yet...

 

I've got a reg. cab xlt 4x4 with the 4.6 3.55 ls rear end and I have noticed the exact same thing. 70 mph gets so much better mileage that 75. I had to go about 50 miles not too long agao to get some gates and other farm stuff and wasn't in a hurry or anythign and it was ll two lane road so I set the cruise at 55 (I also had a ticket the week before so I wasn't going to push it) and I got 21 mpg.

 

Well I am convinced that it was the truck that killed the gas mileage. The temperature was similar, a 200ft elevation over 500 miles (thanks for the statistic mkoser) is not a real factor, and the fuel should have been the same (87 octane). The engine did sound like it was working so much harder but it was only 100-200rpm and 7mph difference. I wonder what would have happened if I had went 85mph and 2400rpm? That 4.6 would have drank enough to get drunk!