In 1982 the average was 4mpg.
Currently, in 2006 the average is 6mpg.
Sometimes, depending on the terrain traveled most frequently, eg; mountains or plains, mpg varies drastically.
The highest average I have witnessed is 9mpg.
The lowest I have witnesses is 1.75mpg.
A military tank with the same diesel engine can average 2 gallons per mile. That is not a typo.

a class 8 highway tractor will vary depending on it brand of engine. caterpillar averages 6.5 to 7.5 cummins averages 7 to 8 detroit averages 7 to 9 and mercedes benz engine averages 9 to 12. some of the mercedes engine have turbos the will spool up to 50 000 rpm. that amazing for a turbocharger of that size. hope his helps you... from David

With the new engines such as the Caterpillar Acert and the Cummins ISX, both over 450 hp, I would say an average of 5-6 mpg. But I've seen as good as 8 mpg and as poor as 4 mpg. It depends alot on idle time and terrain. A tractor that is pulling alot of mountain terrain and off road is likely to have lower mpg than that of a truck that is on level highway all the time.

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.

An average per week of the 3 class 8 semi's I own, is anywhere from 6.3 to 7.6 depending on the route traveled. More mountainous the travel, the less fuel mileage you get. A good average for most trucks is 8 mpg.

The average mpg is 5-9 mpg. Less fuel would be burnt by train travel but it has its own implications as travel time from a to b.

my hubby gets 7.6 in his 99 frightliner century

between 5,5 and 6,5 if you are smooth on the throttle