Kenworth SuperTruck 2Finally, we have two prototype semi-trucks, which happen to have same name, oddly. This one is the Kenworth SuperTruck 2. This highly designed semi is made with aerodynamics in mind, which is why it bears a small resemblance to the Japanese bullet train. This beast uses a PACCAR MX-11 engine and a PACCAR TX-12 transmission like some other semis on the list, but it also comes with a mild hybrid system. Those, combined with improved aerodynamics, gives the truck its impressive fuel economy numbers. According to Kenworth, the semi can achieve a very impressive 12.8 mpg. It achieves this through the aforementioned hybrid system, aerodynamics, and a massive weight reduction of nearly 7,100 pounds. Kenworth developed the truck over a six-year period before showing it off at the ACT Expo in 2024. Thus, this truck is not in production, and you can't buy one. However, truckers' dream of a future with double-digit mpg all the time is getting closer.With the way things are right now, Kenworth's SuperTruck 2 is the second most fuel-efficient semi that we're able to find. That's not bad for a truck that boasts only 440 hp and a 48-volt electric generator. Per Kenworth, the mild hybrid system doesn't even help turn the wheels. Instead, it runs the engine fans, HVAC pumps, and other things that are typically mechanically driven so that the engine can focus on spinning the wheels.