"It depends" on your driving routine. Lots of stop-and-go driving, or otherwise under 35 mph? Aerodynamics aren't going to help you much. Long stretches at sustained speeds over 45mph? More likely to see a return, especially on boxy vehicles and OTR tractor trailers as shown in some of the early posts. On the commercial vehicle side, a full boattail or "Trailer Tails" may show greater returns, but they also have other drawbacks like somewhat complicated loading/unloading, extra weight, and more likely to be damaged in the truckstop parking area. Standard fenders and bumpers have a hard enough time surviving
