A cirumference of 2146mm yields a tire diameter of 26.9 inches – a reasonable number for a 21” wheel so it sounds like you are in the ballpark. Easy way is to just make a chalk mark on the floor beside the tire, then push the bike forward one tire revolution, make another mark and measure between them.
Handy conversions:
1 inch = 25.4 mm
Circumference = Diameter x PI (PI = 3.1416)
Lorin, what size tires do you have on that thing? (Circumference = 83.25cm = 32.8 inches => Dia: 10.4 inches :eek: )