When I experimented with a half tonneau on the back half, the numbers were higher (.5mpg on tank 1 and 1.5mpg on tank 2) than when I had just my camper top on. I only ran it this way fer about 2 tanks when I was in the start of building my aero cap. Not sure how yer side boxes will play into yer results though. Yer half tonneau should be level with yer tailgate.