xkcd2585.py

from pint import UnitRegistry
ureg = UnitRegistry()

def conv_round(base, conv):
    return round(base.to(conv))

def gain(base, conv):
    val = conv_round(conv)
    return (val/base).magnitude

units = (
    ureg['m/s'],
    ureg['knots'],
    ureg['fathoms/sec'],
    ureg['furlongs/min'],
    ureg['kph'],
    ureg['furlongs/hour'],
    ureg['mph'],
    ureg['yards/sec'])

v = 17 * ureg('mph')
e = 45 * ureg('mph')
while v < e:
    print(f"{str(v):48} {round(v.to('mph'), 3):36}")
    v = max(conv_round(v, u) for u in units)
print(f"{str(v):48} {round(v.to('mph')):36}")

Classy.

gain is never used, is it?

Nice!