I recently added a second car to my Fuelly garage and now my signature badge for the first car is incorrect. My Cruze is showing 40.8 MPG on its page, but the badge shows 31.1 MPG.
It looks like the Cruze badge is using: Total Miles (Cruze + Suburban) / Total Fuel (Cruze + Suburban) or
(1726+366) / (42.318+24.592) = 31.098.
OK, that makes sense now. During my first fill up for the Suburban I didn't realize that I needed to select the correct car from a drop down (since I was already on the Suburban page), then I saw that I could move it to the Cruze.