Let me give you a brief example. My engine revs at 38000rpm max. So (rpm)e = 38000 r/min (hpiracing sais 40000 out of the nitro star 18ss but I don''t believe them.LRP z.18 makes 36000 but mine is modified so I''m almost sure that it revs somewhere around 38000 practical). I use 23T pinion (2nd gear) and 43T spur. So T1=23 and T2=43. Also the final gears are Tf1=13 and Tf2=38. Also the wheel radius of my current wheels (rim+tire) is 3.4cm. So Rw=3.4

Vmax = 0.003768*((T1*Tf1*Rw)/(T2*Tf2))*(rpm)e => Vmax=0.003768*((23*13*3.4)/(43*38))*38000 => Vmax = 89.0825 [Km/h]

To convert that in mph: Vmax[mph] = 0.62*Vmax[km/h] => Vmax[mph] = 0.62*89.0825[km/h] => Vmax[mph] = 55.23mph