I'm at 5000', use 87 with up to 10% ethanol and got about 20 MPG when my rig was new, and that went down to 18 MPG as I wore my tires down to 25". With my new 27" Terrabites, I may be getting more than 20 MPG but I haven't gone through a full tank yet on them. I do have a machined sheave installed as well with near-stock weights and a swiss-cheesed airbox lid.
My gas mileage is fully-accurate since I use SpeedoHealer to calibrate my speedo to GPS, stock is off by 2 MPH at around 50 MPH with stock 26" tires. As far as I know, I"m the only person on the 3rd planet from our solar system using SpeedoHealer (it's not like I own stock in it or any of the vendors I support carry it), so I'm curious Wolvyman how you're calculating your mileage as you have 28's installed, which probably puts your MPH/odometer at 2 MPH/Miles faster than actual. Are you using distance traveled via Google maps or something similar or your trip meter?
I do think you'll get better gas mileage just cruising with your larger tires. What's your elevation? Do you have the Yamaha big tire kit installed or are you using stock sheave weights?