looking at the cassette and chainrings I just replaced, I spend 90% of my time on the big ring (50) and somewhere in the middle on the rear (12-25) so probably on the 15 sprocket on the rear. My journey is entirely flat, I'd estimate my average over the whole year is around 17.5ish but occasionally I can average 20 over 12.5 miles, but I do no other cycling apart from commuting, never any longer distances