A quick question here. Has anyone noticed, that installation of a tonneau cover saved you folks some fuel? I have recently bought a cover for my RAM2500, yet didn't have a chance to install it yet. As soon as I install it, I'll post a quick mote and say, how it works on my truck. Yet, I'm curious if anyone had a chance to measure gas mileage with and without a tonneau cover so far?
I think this myth was busted on mythbusters years ago. They tried various theories, such as fitted covers and even driving with the tailgate dropped down. In the end, they found a cover made little to no difference, as you get a vortex effect with air pressure circulating in the truck bed, forcing air to go straight over anyway.
I've noticed similar when I drive with roof down, the air flows straight over the top, I see no fuel efficiency penalty for having the top dropped.
A transportation agency in Canada did wind tunnel testing of varies pick up configurations. With and without the cover, and with the tailgate down.
The cover improved air drag on all trucks.
Putting the tailgate down increases drag because you lose the 'bubble' of air in the bed. Except in the case of the regular cab, long bed truck. Which is probably something only available to fleet customers these days, if at all.