Caps certainly make a difference in MPG. If you are running a pickup without a tonneau cover, the addition of a cap that is the same height as the cab will improve your gas mileage. I remember seeing an article comparing several caps, and there was variation between them. The one that got the best mileage was the same height as the cab, and fit very closely behind the rear window. As a matter of fact, I believe there was a replacement window made by the same manufacturer that slid from side-to-side, so you could access the rear from the cab. Sorry I don't remember which magazine it was in! If I run across it (I never throw ANYTHING out) I'll get back to you. Perhaps CONSUMERS DIGEST did a similar review?