Why not leave a blank area before "MPG" and we can use black vinyl letters to fill in our own numbers or even use a sharpie marker? The marker would be permanent, but I think vinyl letters would peel of if you wanted to change it. Something like bbjsw10 made like this though...........
Or you could make separate ones like...........
30+ MPG
40+ MPG
50+ MPG
60+ MPG
70+ MPG
And so on.