Question 866658
220 rpm means 220 circumferences per minute.
1 circumference is {{{2*pi*21}}} inches.


{{{220(rev/min)*1(circumf/rev)(2*pi*21)(inch/circumf)*1(foot/(12*inch))*1(mile/(5280*feet))*60(min/hour)}}}


Check the units and you should find that the ratios are arranged to give miles per hour.
The strictly numeric part of the expression is/can be:
{{{(220*2*pi*21*60)/(12*5280)}}}