I read up on this forum and on other places about running 17". I was planning on getting 17" Enkei RPF1s to get the immense weight savings from stock, to reduce risk of curb damage with that extra half inch of tire (a lot of places I park have really crappy sidewalks/roads that throw off perception), and to increase ride comfort. I don't care so much for the looks.

To achieve the above goals, I am planning on running tires that replicate stock wheel+tire diameter, with the tires being a little wider than stock. Why does running higher sidewall tires cause greater wheel flex? Is it just because the smaller wheels are weaker (less material) than the larger wheels? I'd have assumed the smaller wheels are smaller on the radial arms of the rims, which wouldn't reduce the strength of the wheel significantly. Can anyone else explain pros and cons of running smaller wheels?

 

Some Pro's of running 17" wheels is they are lighter, the diameter is smaller, so it will be similar to getting 4.77 gears. Also tires are much cheaper for 17" wheels as well.

How wide are you looking to go, 17x8? 17x9?

Are you just looking for lighter wheels for DD, or will you be tracking/AutoX'ing as well?

 

I'm aiming to keep the stock total diameter, which would mean I don't get the benefit of added acceleration through gear simulation. My only acceleration boost would be through the lower initial moment required to roll the lighter wheel/tire combination.

The stock 18" rims are 18x8.5 right? If that's the case, I'm going to aim for 17x8.5 or so to prevent rubbing, and just increase the width of the tires if possible (I think 235 and 245 fits on the 18x8.5 without rubbing).

I'm looking for DD, I may do AutoX but that'll be quite a while in the future. I will definitely appreciate the lower cost of 17" tires. My current ones are the Dunlop 8090 runflats, which go for 250-300 a pop. It's killing my wallet, and probably my back according to a ton of sources.

 

Stock tire is 18x8 +50.

If you went with the 17x9 RPF1's, and got a 255/40/17. That would be pretty close to the stock diameter. If you get 245/40s or 235/40's, your overall diameter will be smaller than stock.

 

Thanks for the info. I'm aiming to keep it at the stock diameter because I want to increase my ride comfort, hence the higher sidewall. Of course, my perception of ride comfort may just be distorted by the harder sidewall of my runflats.

 

Keeping the same diameter limits you somewhat in 17 in wheels, and means tires like 245/45-17 which is still a bit smaller than oem, or 225/50/17 which is about as close as you'll find. I've tried SIX different wheel tire combinations, both 17 & 18 (225/45/18, 235/40/18, 245/40/18, 275x35/18...and 245/45/17, 25540/17) whew!

Consider two things more than any others for ride comfort:

1) the tire sidewall HEIGHT and the tire construction (which is hard to know).

2) using the "measured wheel width" (usually a range with one wheel width called the "measured wheel width" by the manufacturer) or the SMALLER width allowed to get a bit more flex in the sidewall.

Also I've found that wheel/tire weight doesn't play a major role in DD comfort either as long as the wheels stay 20-22lbs or lower, and the tires 22-25 lbs total 40-46lbs.

Right now I'm very happy for a DD with my new 245/40/18 on the oem 8" wheels. They are so much more comfortable than the Goodyear's 245/40/18 I stretched onto 9.5" wheels by an order of magnitude (butt-meter), and about equal to the 245 or 255 on 17x9" wheels although they were NT-01s, a relatively comfortable r-compound.

Alternatively, 225/50/17 should do that in spades pretty much regardless of the tire even, due to the very high sidewall. On the down side, road feel, handling precision, things like that, probably suffer quite a bit depending.

 

^Good info.

But please do not get 225/50/17. That will just look rediculous on a RX8.

 

*EDIT (made a minor miscalculation)
The difference between 245/45 and 225/50 is about 2.25mm (I assume radially, so it's a total height difference of 3.5mm). The stock tires (if I did my math right) are 18" + 4" of sidewall on each side, for a total of 26". With the 245/45 and a 17", it'd be 17" + 4.3" of sidewall, for a total of 25.6". With the 225/50, it'd come out to 25.8" (4.4" of sidewall).

245/45 = 1.5% less than OEM diameter
225/50 = 0.75% less than OEM diameter

Will the speedo then be affected by 1.5% assuming it's 245/45?

With regards to the tire material, I really wouldn't know one from another. However, I'd go with brands I know and tires that have been well rated on this forum, such as the Potenza RE960AS or the Goodyear GTs. With all that in mind, I think 245/45/17 on a 17x9 RPF1 that Fluid Motorsports sells would be a pretty good combination for me.

It shouldn't really affect my DD handling/precision, and I'm not exactly going AutoX anytime soon.

 

math is fun, but this is visual and has all the dimension ---> http://www.miata.net/garage/tirecalc.html

...of course your speedo will be off by whatever dif % from oem. IMO using others opinions is a crapshoot. I've bought tires rated highly and they suck, others rated poor ride quality - ride soft and cushy. Again, even here in the club, the reviewers wheel width, tire pressure, other suspension mods, new or worn when opinion given, it all plays into it and it's all so confusing!

 

Hah! My math is pretty damn close!

And I'm sure it's much better than my stock 18" 225/45/18 with 8090 runflats. I kept hearing people say how the RX-8 is such a great ride, and mine gives me hell on my local roads (albeit my local roads suck *****). I'm just trying to find a solution that A) gives me lighter wheels for general increase in performance, B) is nice rims, C) doesn't cost me an arm and a leg everytime I need to replace tires, and D) can theoretically provide more road comforts than what I have now.