If the 20 inch are the original factory size, then yes, as they have the same rolling circumference.

That said, the narrower tire won't handle the same, so watch your speed and don't push it anywhere near a cornering limit.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue