guys, you just need to know the manufacturer's recommended spec for a given wheel size on their car and then use an online rolling radius calc to get the nearest difference in overall rolling radius when changing tyre profile.
From memory for 17"s on Impreza
classic is 205/40/17 OEM (correction, 215/40/17)
newage is 215/45/17 OEM
correct me if your owner's handbook says otherwise
for the calculator go here
http://www.carbibles.com/tyre_size_calculators.html and scroll to 'Comparing tyre sizes'
HTH