I’m sure this has been discussed at length, but I didn’t see it after a search of the forums.
Reading this thread got me wondering how many crank revolutions it takes to go a (statute) mile. So I did the easy math…
20-incher: 1009 turns per mile
24-incher: 840 turns per mile
26-incher: 776 turns per mile
29-incher: 695 turns per mile
36-incher: 560 turns per mile.
20,168-incher: 1 turn per mile
Obviously the actual diameter of wheels/tires vary a little, so this isn’t spot-on.
Wouldn’t crank length affect it too?
Hmm … interesting …
Assuming a revolution frequency of 2Hz gives a speed of 11.48 km/h for 20inch 
Crank length can affect the speed of a rider, but won’t affect the number of revolutions required over a given distance.
Shorter cranks mean your legs don’t have to travel as far though, that’s true.
Why? I think he assumed that the circumference of a 20inch tyre is 20inch * 3.14 = 159.512 cm and a mile are 1609.344m => 1008.91 revolutions are needed
Right. Except for the metric conversions…didn’t include that in my process!
One of my theories is that the energy required for a particular day’s ride correlates with the number of crank revolutions. A distance ride on a 36" with 150mm cranks is easier than the same ride on a 29er with 110 mm cranks, not because you can go faster on the 36 (which you really can’t), but because you have fewer pedal revs. For the same reason, a geared 29 with 150mm cranks will be less tiring than an ungeared 36er.
But then I’ll be the annoying person to bring in some real-world information:
Tire diameter is usually pretty far off from the exact number of inches printed on the side. It’s actually pretty amazing how far off some of them are.
Then there’s the annoying fact that we mostly can’t ride in a straight line. Do we count this in our calculations? My short answer would be no; that the correct # of revolutions to cover a mile has covered a mile. It might be a squiggly mile, but a mile it was.
Lastly I’ll add that I am happy to have a geared hub, to cut down on those big numbers of revs!
I agree with John. I actually measured my two favourite Unis the other week.
My 24" QU-AX Muni (3" Duro) = 26" actual Diameter
My 29" UDC Road Uni (1.75" tyre) = 28" actual Diameter.
So for me, the speed difference between my 24 and 29 isn’t as much as you’d think 
Yes, when the crank length is greater than the radius of the wheel. 