MikeW

I was thinking that the Rx-8 should probably have 215/50 17 as its snow tires, if it has 13" front rotors.
215/55 16 would work on the 12" front rotor equipped cars.

And after the OEM Bridgestone Potenza RE040's wear out, I would replace the 225/45 18 with 235/40 18 tires. A little more appropriate for an 8" wide wheel.

Comments, please.

PatrickB

*Shrug* Not being a winter driver, I can't help you much here. It is worth noting, however, the Bridgestone Blizzak tires are available in the standard 225/45R18 size, so you don't necessarily have to ditch the 18" rims.

snow_tires

winter driving implies marginal conditions.
the extra rubber between you and the road comes in handy.
and in heavy snow the narrower tires will handle better. so i would go with the smaller rims too.

not to mention that trashing 18" rims costs more than replacing a 17s or 16s.