Depends on how wide the rims are.
Since the OEM 21" size is 325/30R21, I definitely would not get smaller since your car would look like rolling out of the hood.
Here a calculator to match similar tire sizes with the larger rims.
http://www.techcons.at/index.php/abrollumfangrechner
345/25R22 would be a close fit. 0.36% difference.