I've noted every fill up since I bought the bike at 28955 miles on the odo. It's now just over 101k with an average MPG of 51.36 over all those miles. For a single spark plug Wee, with a K&N air filter and a Scorpion exhaust with the most free flowing (read: loudest) baffle in this is perfectly acceptable. I'm not sure why I still keep track of it all, it's probably just my little OCD, because I don't really care what the MPG figure is as long as it stays roughly the same. If I wanted high MPG I'd buy another Yamaha SR125 Custom and rag it to within an inch of it's life and get 100mpg average still. I've tried taking it slower to improve MPG on the Wee back when I was still doing the ~186 mile round trip everyday for my commute. That didn't last long, I was bored out of my skull on the motorway within about 4 days.
Funny thing is that if I load the panniers and put the good lady on the back I get better MPG, I put this down to taking it easier when she's with.