The most fuel-efficient motorcycle among the Harley-Davidson and Yamaha lineup is generally the Yamaha models. Yamaha motorcycles are known for their high fuel efficiency, particularly the Yamaha FJR1300 and Yamaha R6. These models are designed with lightweight materials and advanced engine technologies that optimize fuel consumption.

The Yamaha FJR1300, for instance, has a fuel efficiency rating of around 45 miles per gallon (mpg) on the highway, which is significantly higher than the average Harley-Davidson model. The Yamaha R6, a sportbike, offers an impressive 58 mpg on the highway, making it one of the most fuel-efficient sportbikes available. This high fuel efficiency is attributed to the Yamaha's compact, lightweight design and its fuel-injected, four-cylinder engine.

Harley-Davidson, on the other hand, is known for its powerful, V-twin engines that are less fuel-efficient compared to Yamaha's inline four-cylinder engines. The average fuel efficiency for Harley-Davidson models is around 30-35 mpg on the highway, which is lower than the Yamaha models mentioned.

Data from the EPA (Environmental Protection Agency) supports these claims. The EPA rates the Yamaha FJR1300 at 45 mpg on the highway, while the Harley-Davidson Street Glide is rated at 31 mpg. The Yamaha R6 is rated at an impressive 58 mpg on the highway, while the Harley-Davidson Sportster 1200 is rated at 39 mpg. These figures highlight the significant difference in fuel efficiency between the two brands.