What is the fuel mileage for international semi trucks?

The standard answer is 6mpg, but the real answer is "it depends on a lot of things."

Two days ago I hooked up to a load of Budweiser and pulled it through some really hilly terrain with an International 9400i. I got 6.2mpg. After I got rid of that I went to another place, loaded with a ton of molded styrofoam--it filled a 53-foot trailer--and got 9.7mpg over the same terrain. And once I pulled a load of car tires from Atlanta to Dallas--a fairly flat route--and got 11.8mpg. This is a truck with an automatic transmission, economy gears in the rear end and a 435-hp engine. If I had a 9-speed manual, hill gears or a smaller engine I'd get worse mileage. You're thinking, "don't you mean bigger engine?" Nope--a smaller engine would have to struggle more to get up the hills, and when you stress one of these engines it responds by sucking down diesel. The current fuel mileage champion gets over 11mpg ALL the time, and he's got a 600-horse engine. He barely has to crack the throttle on that thing to make it move.