Cap vs. Tonneau MPG

[P M]

If really concerned with gas mileage the ST isn't the right vehicle to begin with. No big mpg difference should be noticed due to the cap.

Well, I'm not hauling all my stuff in a Prius, and the ST is what I have, so I am at least interested in the impact of MPGs. My question was for those who have one, and while I don't think it will make a big difference, I was at least curious if the change would be up or down. That's all.