I got my 2008 Mazda Speed 125/70D17 spare off of Ebay for $95 with shipping. It fit both front and back locations and I took it for a test drive on the front. The Mazda Speed spare is a 17", like the original SC430l except the tire is about 7% smaller. My Speed spare has a 1,653 pound load rating. The tire fit right into the plastic tray in the trunk, the same tray that contains the jack, tire iron, etc. It does take up quite a bit of trunk space and I'm considering a continental kit. The SC's had a 5-114.3 bolt pattern and I have found that many other cars also have that same pattern. If you decide to use another spare, the other issue is "Offset" and "Back Spacing", and you can get that info off of sites like TireRack.com. I hope this helps.
