i got these specs from the Toyo tires website....
my stock G20 came w/ 195/60/15's... heres a comparison with the best match for each wheel size...
tire size diameter (inches)
195/60/15 24.2
205/40/16 22.5
215/40/16 22.7
205/45/16 23.2
205/50/16 24.0
225/50/16 24.8
245/50/16 25.4
205/55/16 24.8
225/55/16 25.6
205/40/17 23.5
215/40/17 23.7
235/40/17 24.0
255/40/17 25.0
215/45/17 24.4
225/45/17 24.9
245/45/17 25.7
215/35/18 23.9
225/35/18 24.2
225/40/18 25.0
235/40/18 25.2
245/40/18 25.7
275/35/18 25.6
225/35/19 25.2