different sized rims f&R?
http://cgi.ebay.com/ebaymotors/Merce...spagenameZWDVW
http://cgi.ebay.com/ebaymotors/Merce...spagenameZWDVW
it's just a matter of the reducing the sidewall profile of the tyre to compensate for the increased rim size, thus keeping the rolling radius as close to stock as possible ( a difference of up to 2.5% is typically acceptable)
for reference the clk dtm supercar uses 20" rears and 19" fronts







