To get great mileage one has to have serious control. I figure on the trip I got good mileage on, I averaged 55 to 70 for speed, and there were few stops I had to deal with.
Meanwhile, even if I drive carefully in the city, the mileage averages in the mid/high 30s in no time.
Anyone not getting close to 50 MPG with nearly all 'highway' time and going under 75 (yeah, I know . . . harder than hell to do for a long time) might either have something sucking away their fuel efficiency or maybe the system isn't calculating correctly. I've watched the calculation for gallons/miles covered as well, and the numbers match quite well. I don't expect perfection. There' no guarantee I'll put in the exact amount each time.
No modifications were done to my bike. The slightest bit of road romping though can seriously deflate those mileage numbers.