[Paladin1]

The only run flats available in 21" tires - that I'm aware of - are either Summer tires or snow tires. There are no all season 21" run flats made currently, just for your planning purposes. If you plan on driving in Winter weather part of the year, you'll need to plan on either getting snow tires to swap out, or get all season non-run flats, in which case a spare might be handy. If you drive in Florida all year, Summer run flats should be fine. I haven't noticed a significant difference in ride comfort with 20" run flats and 21" non-run flats in a previous car, both with air suspension.