関数 Unit(expr) は引数 expr を単位として扱い、 標準的な形にした結果を返します。

with(Units[Standard]):

Warning, the assigned name polar now has a global binding
Warning, these protected names have been redefined and
unprotected: *, +, -, /, <, <=, <>, =, Im, Re, ^, abs,
arccos, arccot, arccsc, arcsec, arcsin, arctan, argument,
ceil, collect, combine, conjugate, cos, cot, csc, csgn,
diff, evalc, evalr, exp, expand, factor, floor, frac, int,
ln, log, log10, max, min, normal, root, round, sec, shake,
signum, simplify, sin, sqrt, surd, tan, trunc

3 * Unit(ft/s);

convert(32.0, units, ft/s, km/h);

convert(32.0*Unit(ft/s), units, km/h);