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As a current semi driver I can offer a little insight here. Tractor trailers are becoming more and more fuel efficient in tiny leaps and bounds. But weight and road conditions are key factors in how well it does on its run. Main factor, is gearing matchup with engine rpm, and finally rear axle ratios. The common transmission is the Eaton 10 speed, with 13, 15, 18, and even a few 21 speed setups available. Add in the new automatics (turns head to not puke) and the possibilities on gearing are many. Add in the rear axle ratio which can run from 3.55 to 5.23 (other ratios do exist but not certain on the numbers) and again, the engine is allowed to run more efficiently in its powerband. Idle is normally between 500rpm to 700rpm, with normal operating rpm between 1400 to 1900rpm. The Freightliner Columbia that I run in, typically gets 6.5 to 7.2 mpg, with 4.8 being the low so far, and a high of 7.7mpg. Add in the twin 100 gallon tanks (actually only hold 90 gallons each due to cap angle, and internals) and the range can be pretty good. Now another made the comment why cannot cars get 80mpg if the tractor-trailer can get 5-7 mpg. Well, dig hard enough and there have been vehicles that have gotten that kind of mpg. Sadly, the automotive gurus and makers decided that economy was not a selling point, but that speed and horsepower is. So, when Mopar actually had thier 2.2L four cylinder actually break 90mpg they hid it and the specs for it. GM had a 5.0L V-8 back in 198x break 40mpg and guess what... they hid it as well. Recently some inventor in Iowa (if memory serves, cant find the article at the moment) made their vehicle mods to allow it to run well above 50 mpg. It is not a matter of why cant a car do it, it is more a matter of why does big business wish to hide and deride the engines and vehicles that can.
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