Took a roadtrip in 09 Plat 5.4, 4x4, 6.5 bed, 3.55 axle from New Jersey to Florida. Truck had 11,900 miles when we left. On way down we had some really odd gas mileage numbers, very low. Started out getting little over 18 on dash computer. As the trip went on going down, ( got fuel in N.C at Petro) I could watch the gas mileage keep going down . Outside temp was upper 90's and we were running 70 to 80 keeping with traffic. It got down as low as 15.7 on computer.

Here is the only thing I did different with the truck. I had made a bug screen as was suggested on here for the bugs in the south. I know I was running fairly fast most of the way, but it seemed lower than I had been getting on shorter trips at home. I was looking for high 18's or 19 with no stops. It was driving me nuts when it hit 16 avg.mpg. So somewhere on the Florida Turnpike I took bug screen off. Don't know if it was a factor but mileage was up in the 18 + range the rest of way down.

Was watching very carefully on the way home from Siesta Key, Sarasota, FL area. Was running 70 mph plus all the way into Va. and averaged a solid 18.0 mpg. As some have said here computer avg. is off .5 to 1.0 lower versus hand calculate miles.

Having cruise on or off really was no different. Just some observations. I too was reading guys hitting 21 plus with the 5.4, but was not expecting nothing over 19 to 20 if you can run at 65 to 70 mph. Did really like how long you could go before fueling. One tank was 575 miles with 30 plus until empty. It was a nice comfortable ride all the way around in the truck.

 

well the only thing i can think of is when you took off the bug screen you got more airflow through radiator and less resistance makes sense to me. It appears it make it more aerodynamic so to speak