The only reason I care about MPG on the GROM is because of range. I do some 10 hour rides and am away from gas stations for many miles. My grom is TOTALLY STOCK. My MPG goes way down when I run wide open throttle at 55 MPH for long periods of time. I find that if I let off the throttle a bit, just enough to get it out of its WOT FI mode, but NOT reduce enough to loose power, my fuel mileage goes way up. So my technique is, pin the thorottle, let off as much throttle as I can without losing any power, and run it there. Same speed, and a lot more range
The other thing that will change your life on a GROM, get a GO CRUISE throttle lock, the aluminum V2 version. For 30 dollars, and makes a world of different when running constant power settings. Installation is 1 minute, and you can take it completely off the bike in 30 seconds when riding in town. That little device change my life !!!