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I've been spending the last few days trying to get my head around what wheel and tyre sizes to get for street use. Finding a set of wheels that will look nice on the car is hard enough but then trying to figure out if they come in the right size and offset, if the tyres that fit them well aren't really rare/expensive and don't increase the rolling radius too much. my head is about to explode.

I've kind of decided on getting 18x8.5 wheels with 225/40-18 tyres but I'm worried that the extra diameter and increasing the weight per wheel by 1kg or more will kill the fun factor of the car and dramatically change the way the car feels...
Has anyone got a combo like this and can comment on what it did to the feel of the car?

Also, as long as the ATO is kind to me, I will probably also order a set of Enkei RPF1 17x9 with 245/40-17s for track days. It seems like overkill to me but I'm attempting to prep my car for a supercharger kit that I keep blabing about and is possibly a figment of my imagination.
Thoughts? Comments?

In NSW wheels can only be 1" wider than std and track only 1" wider.
That will limit you to 8" with a max IIRC of 45 offset.
I've gone with 17X8 PF01 with a 50 offset
http://www.good-win-racing.com/Mazda-Pe ... 10126.html

I'll be hanging 235/40/17 off them to give near as dammit the same diameter as std.
Go to an 18" wheel and you'll be forking out for 235/35/18 which will see the bank balance seriously hurt to say nothing of your kidneys and fillings

The 17x9 RPF1 really does sound like the ducks guts for the track as soon as you get over 100RWkW under the new rules.

I recall someone posting about 18x8.5 being legal if they were engineered (Charlie Brown?). The only reason I'm wanting to go 8.5 wide is because I like the BBS CH-R's which come in 8.5 min IIRC. If they end up being stupidly expensive, I'll drop down to 8" in something else...

So how did the extra grip affect the fun factor of the car? Does it feel any slower or less fun?

Madanark, I don't know of anyone in AU running 18 x 8.5 wheels, and there are a number of reasons for this.

Yes you'll be able to get them engineered when you get the S/C done but I'd go and have a chat with the approved engineer that you’re going to use for the S/C to ensure that they'll pass them before you head out and buy the rims. Check your clearances against the standard tyres remembering your stock rim has a 55mm offset. The standard 205 x 45 x 17 tyre is 616mm in diameter. A 225 x 40 x 18 tyre is 637mm in diameter and if you chose a 55mm offset would be 10mm closer in both directions on the inside. Check your lock clearance and remember vertical travel.

Performance wise the car will be slower than using the stock wheel. Besides the increased weight in the tyre & rim that needs to be turned there's a 5.5% increase in rolling diameter. If you have been reading Miata.net forums remember that the American's won’t notice the drop in performance like we would as they have a 4.1 diff ratio to our 3.7.

Your choice of 17 x 9 with 245 x 40 tyres for track days is a bad call before supercharging.
First up they don’t qualify in the modified 2B class where every other modified NC’s runs as they are over the width limit by 1”, so you’ll need to run in the SV class which has no limits to the modifications you do as long as its road registered. This class is for those with plenty of $$$ and will be the class you go in when you fit the supercharger.

Secondly you need to be very careful in picking the correct offset to clear the inside body work. My 16 x 8” rims with a 50 offset running 225 x 45 R-Specs (608mm dia.) just touch on the inside of the body work over the Wakefield bumps. It’s also lowered around 40mm which decreases clearance. A 17” x 8” rim with 245 x 40 tyres is 628mm dia., so would be 10mm closer at the top and 5 mm closer to the inside than my tyres and would hit the inside bodywork for sure. You’ll probably need a minimum 35 offset to the rim. This then produced the next problem, the guards will need to be rolled to get clearance on the outside (mine are slightly on the front to clear) and most probably cut to fit flares to cover the tyres. If you measure 40mm out from your standard tyres you will see where the 245 tyre and 35 offset will come to on the car.

Another thing I’d like to point out is that the quickest NC around Wakefield uses the stock NC rims and runs 205 x 45 R-Specs which backs up the theory that as soon as you throw big wheels at the NC, or any MX-5 for that matter, the weight kills the straight line performance.

Time for some more head exploding time.

Time to revive an old thread. I've changed my mind on wheels again and I've got a quote from Tire Rack for them but they spooked me when they said "These wheels are not a listed fitment for your vehicle. You will be responsible for any fitment issues."

I know I'll be up for some guard rolling but I'd like to confirm that is the only issue.
The wheels are Black Enkei Raijin's in 18x8 5x114 ET45 with a set of KU36's 225/40R18.
I'll also order a set of hubcentric rings 72.6mm outer and 54.1mm inner.

So do I have all my bases covered or am I missing something?

they will fit fine. I run 17x9 +40 with 225 45. fitment wasn't hard. only needed gaurd rolling. it doesn't scrub. and I am very low. so those wheels you want will be fine.

Fitting bigger wheels and tyres may also place a greater load on your wheel bearings. So much I have gleaned from past experience.
(VK Brock Commodore, Irmscher wheels...crunch crunch crunch)
or TC Gemini...ROH mags + Goodyear NCTs. Heavy buggers, Same thing after 12 months.

cheers
Chris