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Message:: [QUOTE="Bret1984, post: 11950250, member: 297040"]It's not physics though. Fuel efficiency has more to do with RPM's than speed. Where that "sweet spot" is on the RPM's where peak efficiency is achieved is going to vary from truck to truck. In top gear the optimal RPM range for peak efficiency may be around 60mph or it could be closer to 70mph depending on how the truck is geared. Physics effects MPG's to the extent of aerodynamics and weight. As for the optimal speed that will be the speed at which optimal RPM's are achieved in top gear for highway driving. So no, slower is not more efficient which is why the city mpg rating is going to be lower than the highway mpg rating on any vehicle despite slower speeds during city driving. If the slower you are the more efficient you are were true then city mpg ratings would be higher than highway mpg ratings.[/QUOTE]