[instarx]

Some misconceptions from previous comments:

1. lots of gears on trucks are not there to give good mileage - they're there so the engine can move the truck at all.

2. Low RPMs of diesels do not translate into FE. Even at 1,000 rpm fuel can be pouring into the cylinders to produce power and torque.

3. all things being equal, diesels engines get better mileage per gallon because diesel fuel has 10-15% more energy per gallon than gas - not because diesels have high compression.

Trucks are more efficient than cars per pound moved because of simple physics. It simply takes much less energy to keep even a large load moving at a constant velocity than it does to accelerate it (Newton's First Law, and F=ma). If engines on trucks were made large enough to accelerate 80,000 lbs as fast as a 3,500 lb car they would get about 0.1 mpg (I'm guessing).

Conversely, if car engines were sized proportionally to truck engines cars might get 200 mpg, but no one would drive them because you would have to shift gears 10-15 times between stop lights to get the car moving. If your car engine were sized like a truck engine, a 3500 lbs car would have an engine of 20 cubic inches (0.3 liters)! That's less than half the size of a riding lawnmower engine.

So, trucks use less fuel per pound moved primarily because:

1) it doesn't take 20 times more force to keep 70,000 lbs at 65 mph than it takes to keep 3,500 lbs at 65, and
2) your car is hugely overpowered compared to a truck.