Quote Originally Posted by NTFDAY
I agree with Denny. In stop and go traffic gas mileage will suffer, but as Denny said cruisin' down the interstate at 60 mph you'll get better mileage with the ac on and the windows rolled up. Pickup trucks with the tailgate up and no bed cover will also cut your mileage cruisin' down the interstate at 60 mph.
tailgate down cuts gas mialage. with the tail gate up, there is a wind force inside the bed up to the top of the cab ( caused by a vacume at the base of the cab in the bed ) that causes the passing air to be forced above the cab and helps areodynamics, with the tailgate down, this wind force is gone, and causing it to lend no help to the aerodynamics. saw this 1 on mythbusters too. they used a model of a F150 in a small wind tunnel.