I'm running 4" wheels on the feather that I'm building at the moment, but geared more for speed and silly traction than anything else - so take my numbers with a pinch of salt as I'm one of the crazier people here.

Mine is geared for 20MPH - which is over the top, but what I have space for. Shaft input of 8600RPM with a 5:1 reduction. A good middle ground is the 10MPH mark!