This is my second filling up of GPMINI, so I know it was full before & after … I did exactly 400 miles on the tank, and filled 13.86gallons for an MPG of 28.8; the OBC calculated 28.6 which is pretty close.
Obviously, I’m not yet using the GPs full performance!
M&I / GBMINI 
I was actually wondering about this.
One thing you need to consider. You will soon be crossing the Continental Divide. So far you have been going “up hill” and once you start “down hill” your mileage should improve. Only problem is that as the days get longer the foot gets heavier! LOL