Actually, as speed increases, you tongue load should stay static. Wind may have some effect on how the load 'feels' while towing but the weight itself should not vary at all. H&H has both a round nose model and a V-nose model. From our testing, the V-Nose trailer does tow easier and can add as much at 1MPG to the average tow vehicles mileage while towing. This is an excellent Question! The three main issues when towing for the best possible return on fuel spent are, weight, drag and aerodynamics in general. As a rule of thumb, an enclosed trailer, carrying the same cargo weight as on an open deck trailer will weigh 10% more, which does have to have some sort of effect on mileage, as the more weight you carry, the more energy or fuel it takes to move it. But, a smoothness of an enclosed trailer offsets some drage properties and allows for a 'smoother' hole being punched in the wind as you tow, which compared to open trailers can be a plus. And Aerodynamics plays the final role. I don't think anyone here could argue that an H&H 20' round top V-Nose trailer would not have a better Aerodynamic 'faceprint' in the wind than a flatbed trailer towing an automobile. From my experience, though, using a Duramax dually to tow identical loads from the Midwest to California and back on both open and enclosed H&H trailers. The open deck trailer usually got slightly better mileage, but that cost savings was usually offset by having to re-clean the vehicle on the deck of an open trailer once I arrived. Do you want to save the most fuel while hauling cars, regardless of any other expense? Buy an H&H EXA aluminum speedloader. It will weigh the least, punch a reasonably clean hole in the wind (especially when fitted with our car guard fairing) and tow the smoothest, even hauling full sized autos behind 1/2 ton trucks. Do you want your ride to arrive as clean as when you loaded it? Do you want the ultimate security while on the road from theft of flying debris damage? Get an H&H CL series enclosed car trailer. But, if you want to try to get the most of both, call us and we will quote you our new all aluminum framed CL series enclosed trailer. Hope this helps Craig Hull