In 1973, the feds estimated that semis got about 5.6 miles per gallon of diesel; today's estimate is 6.5 mpg, although different trucks get fuel economy in a range from 4 to 8. Going up a steep hill, a truck's mileage might drop to about 2.9 mpg, while going down the same hill will raise it to more than 23 mpg.read more >>