Some vehicles get pretty much the same mpg at 65 mph as at 50 mph. Usually a vehicle with a larger engine, tall gears and good aerodynamics will be in that category.

Econoboxes usually lose a lot of mileage at higher speeds because they are geared low to make them feel quicker despite a low powered engine. The Toyota Corolla gets better highway mileage than the Toyota Yaris despite being bigger, heavier and having a larger engine for this reason.