RX8Club.com - Tire facts

this is how they have theirs:

I finally compiled the most completed info I can come up with for wheel and tire fitment questions that everyone has. I mapped this entire post out so that you can pick a rim size you want and the rest will follow, including tire size.. I worked out this post so that you guys can decide on a tire size and it will point to you of what rim specs you need to fit onto the car.. as for brands and types of wheels and tires, well, that's a totally different thread on its own.

-GT's bolt pattern is 5x100mm, NOT 5x114.3. Only the 2005+ Impreza WRX STi is currently running 5x114.3mm. Stock GT wheel is 17"x7" +55mm (still need full verification if it's different from model years), but it's been confirmed for 05' rims on post #1522

-OBXT bolt pattern is 5x100mm with +48mm offset (weighing around 20.5 lbs per rim). OBXT wheel is 17"x7".

-2.5i and 3.0R models will need confirmation on stock rim sizes.

-Center bore must have to be 56mm or larger (if the bore is larger than 56mm, use hub centric rings to fit properly to your wheel's center bore, but not always necessary if you have wheels with conical lugnut seating, the wheel will eventually center itself, but that's your call of how you want to go about that).

-SpecB, OBXT, and 3.0Rs can use shorter tire sizes from the GT. But be sure to check and compare the load capacity of the tires. Try to use the same tire load capacities if you can within +/- 100 lbs. You can certainly always go over, but not under (as this requires for you to run higher tire pressures and this could cause more of an issue for tire wear).

-Lug nuts threading are 12mmx1.25mm. Open or Closed End lugs will depend on the mounting thickness of your rim, usually, closed ends (like the stock ones) are fine.

-Here we go, here are your choices for rim widths (regardless of it being 17", 18", 19" or 20" rims):

{offset in BOLD are the closest possible offset to OEM's scrub radius, you'll notice that once you go past 8" wide rims, you will run out of room on the inboard side to put/stuff any more of the wheel/tire to maintain the OEM scrub radius.. 8" and up will increase positive scrub radius}

6.5" (+34mm to +60mm offset, +55) *for 2.5i only
7.0" (+37mm to +57mm offset, +55)
7.5" (+40mm to +54mm offset, +55)
8.0" (+43mm to +51mm offset, +53)
8.5" (+45mm to +48mm offset, +45) *will need to roll the fenders if tires are wider than 255
9.0" (+43mm to +45mm offset, +43) *will need to roll the fenders if tires are wider than 255
9.5" (+38mm to +40mm offset, +38) *will need to roll the fenders if tires are wider than 255

and it also depends on what kind and size of tire you want to run as well (and that can get messy). But to be on the safe side, here is the tire widths you can put on each rim:

195mm = 6.5", 7" and 7.5"
205mm = 6.5", 7" and 7.5"
215mm = 6.5", 7", 7.5" and 8"
225mm = 7", 7.5", and 8"
235mm = 8", 8.5", and 9"
245mm = 8", 8.5", and 9"
255mm = 8.5", 9", and 9.5"
265mm = 8.5", 9", and 9.5"
275mm = 9" and 9.5"
285mm = 9" and 9.5"

Once you decide on the rim size, the tire width you want to run, here are your final tire choices. Note that not all tires are built the same way, so sometimes you'll find two different tire models with the same size (225/45/17), but one can fit on a 7" rim, while the other one will need a 7.5". You'll have to look around the manufacturers' specs on sectional width of the tire.. normally you want your rim width be no more than 1.5" less than the tire's sectional width. For example, if you have a 8" wide rim, your tire's sectional width shouldn't be more than 9.5" or you'll have to deal with the flex on the tire sidewalls. Most performance driving set ups are 1" difference (ie. a 8" wide rim with 9" wide tire). Below I listed for you by rim diameter of which I recommend running with some notes on each tire with notes to go along with them as well:

16 in. rims (for 2.5i, 2005-on) only:

195/60/16 (great for snow rally or snow use)
205/55/16 (OEM sizing which is also good for some non-tarmac surfaces)
215/55/16 (oversized for the rim protection, but very sloppy sidewall handling)
215/50/16 (wider footprint used for more daily driving without harsh gas consumption)
225/50/16 (great for autocrossing, still usable for daily driving all year round)

17 in. rims (for GTs 2005-on):

205/40/17 (only for SUPER FAST acceleration and torque; for autocrossing only, do not use for daily driving)
205/45/17 (only for FAST acceleration and torque; for autocrossing only , do not use for daily driving)
205/50/17 (good for snow rally)
215/45/17 (stock)
225/45/17 (good autocross performance tire size for gearing, more mph per gear)
235/40/17 (good autocross performance tire size for torque, more acceleration)
235/45/17 (tallest tire you could use under the GT due to possible suspension clearance issues, best used with an 8" wide wheel)
245/40/17 (good autocross performance tire size for traction, will need to have a flatter alignment to make use of the grip)
255/40/17 (maximum grip, good for track days, will need a 9" rim to make use of the contact patch and a good alignment as well).

18 in. rims (for GT 2005-on):

215/35/18 (only good for autocrossing use, do not use for daily driving)
225/35/18 (only good for autocrossing use, do not use for daily driving)
215/40/18 (uncommon tire size, close to OEM overall tire diameter, good for daily driving)
225/40/18 (typical sizing, most popular size, GREAT for daily driving)
235/40/18 (very large, but great for pot-hole city driving, may need fender rolling)
235/35/18 (rare size, but closest to OE tire sizing to keep the spedometer reading correctly)
245/35/18 (common autocross, performance size, very good for daily driving)
255/35/18 (very good daily driving size, even better on the track)
265/35/18 (very good daily driving use, but the car wont be able to use all 265mm of tread for the track)
275/30/18 (tallest tire you can mount without fender clearing issues on a 2" lowered car if you get the right offset)
285/30/18 (aka, the "STREETS OF DREAM" setup... the widest you can go.. this is the next tire size I am going for and a rim to match it)

18 in. rims (for Spec B.2006-on, 3.0Rs 2008-on):

225/40/18 (common tire to use and gives a better acceleration since it's 0.5" shorter)
235/40/18 (rare size to find, but near stock height for proper gearing)
245/40/18 (popular tire size for the larger cars)

19 in. rims (for GT 2005-on):

225/35/19 (common typical sizing for 19" for our cars)
235/35/19 (tallest tire you can mount without fender clearing issues on a 2" lowered car if you get the right offset)
245/30/19 (rare size, but also tallest tire you can mount without fender clearing issues on a 2" lowered car)

Use these at your own risk, but to my knowledge on a lowered car with a small-spring perch type coilover, it should work and ride fine. The most you'll probably have to do to the fenders is to just roll the lip of it in to give yourself another 5mm of clearance. I hope this helps.