Eddie,
There are a lot more variables that effect your gas mileage than just the miles and speed. Some are your driving habits that include how fast you accelerate to cruising speed, the terrain, the wind, the temperature, Tire inflation, the traffic around you, how many times you had to slow down or stop and then speed up again, the gasoline blend, and how much weight was in your truck, etc Also, most Sport Tracs do not get their best mileage until they get about 10K miles on the odometer.
Each has a very small effect on mileage however collectively they can make a big difference in mileage.
You must consider that some people here only get about 12 MPG on the highway and others get 21-22 MPG. So there are a lot of variables that effect the mileage, but you are getting better mileage than some people, but not as good as some others. I get about 20 MPG on the highway cruising at 75-78 MPH with little or no traffic, carrying about 500 lbs of luggage, and in the heat of summer with the AC going full bore. I have gotten over 21 MPG on a few tanks of gas and as low as about 18.5 MPG going through the hills of western Kentucky. I do not make any conscience effort to save gas, or I guess I could do better.
...Rich
