I have no drop in MPG with top case, but I do when I run with the side bags, so I run mostly with the top box. I get the worse MPG when I have neither on, but then again, whadaya ya expect when on the track??? I commute 80 a day so MPG IS important to me, but when I'm out having fun not so much. My best MPG is when SWMBO is on back, much smother inputs, speed limit 99.9% of the time. Based on this, I would say the top box does not add to the decrease in MPG, rather, the side bags do. Only 49400 miles, so my estimates may be off, but I have an overall average of 41 MPG for the life of the bike, the vast majority sans side bags, with the Shad 46 mounted.