People are always so quick to brag 7 and 8 mpg. The reality is that if I gave you enough fuel to cross the US and figured your predicted mileage on 7 or 8 mpg, you would not even come close to making it there. If you have a perfectly still day, or a tailwind, and you also have an aerodynamic truck, and you also have a light load, and you also drive slow (60mph) with the cruise on, you will average about 6 to 6.5 if you are lucky. You may even touch 7 mpg if you are really gentle and coast some also. Fuel surcharges pay on 6.5 mpg, but the reality is that you cannot average that. The days when you fight the wind, or fight terrain, or you have a heavy load pushes your mileage down to the 5 mpg range, so you end up with an average for the year of high 5s or low 6s, many dollars below the surcharge.

Truth and reality which most people don't want to face because they look better if they yell out a high MPG figure, you will only average 5.75 for the whole year in a semi truck, unless you have a very careful driver that does not idle or anything else wasteful, which can allow you to average up to 6.5. I am ASE certified in Diesel and electronics for heavy trucks, and I look at a lot of ECM generated MPG, figures, the highest I have ever saw was over 7 with no trailer on a short trip. Usually they are high 5's to low 6's.