Ideal sizes for winter tires to fit 18" OEM's?
 

Can I use 225/45 for all four? If not, why, and which should I use instead?

Oh and would 205/40 work? There's a great deal on a set of these out there.

225/45-18” snows front and rears could work, but here are some things to consider:

1) If your car is equipped with TCS or VDC (and, TCS is included in VDC equipped Z’s), these devices expect a staggered height (fronts having a smaller overall diameter than the rears). If you go with the same height (overall diameter) front and rear, you may experience unwanted engagement of TCS (but that’s only the case if your car is equipped with TCS).

2) 18” wheels with snows is a poor choice. Snow tires in this class are very expensive, and do not provide the flex and traction you get with 17” wheels and snow tires for this wheel size.

3) A better wheel/tire setup for winter driving is buying inexpensive 17” wheels on which you mount Front 225/50-17” and Rear 225/55-17” tires.

--Spike

How noticable would this be?

The Stickys above describe the problem.

If you depart from the correct overall diameter stagger, you could experience a loss of power (TCS killing power to the rear/driving wheels). If that happens in the wrong place at the wrong time, it could put you in harm's way.

--Spike

Members like the 17-inch OEM wheels because the are inexpensive. You can get 4 for $200. For snow tires, thin is in. The proper 16x6 wheels will also be good for snow tires.

Alright, so would 235/45 work for the rears?

If they are on 18" wheels, It should be close enough to work on the rears.

--Spike

Thanks. So as long as it's 45/18 for the back, I'm fine? Or would I need 40's in the back instead if I go over/under a certain width? How about 35 or 50, are they ever used?

Not sure you understand. The second number is the aspect ratio, and it isn't a physical measurement. You need to respect the entire formula, which is width/aspect ratio-wheel size. The important measurement is the overall height. For example, 245/45-18" is the same overall height as 225/55-17" and as is 275/40-18"

--Spike

Alright, and how do you calculate this?

The metric tire sizing system represents a tire size as:
[section width (mm)] / [aspect ratio] - [rim diameter (inches)]

For example: a tire size 225/45-18 is 225mm wide, has an aspect ratio of 45 and fits an 18" rim (wheel).

The aspect ratio (the second number) is not a measurement. The aspect ratio represents the tire's sidewall height as a percentage of the tires width (section height divided by section width). In the example above, the sidewall height is 101mm (101/225=.45).

The aspect ratio is important since that (along with the wheel size) determines the overall diameter of the tire allowing you to match your stock overall tire height (diameter). For example, a 225/45-18 tire's sidewall height is 101mm, and a 245/40-18 tire's sidewall height is 98mm. So, both tires have nearly the same overall diameter and would fit the front of a Z.

Overall diameter OEM Bridgestones Potenza RE040:

225/45 ZR18 25.9" (front)
245/45 ZR18 26.7" (rear)

--Spike

Thanks for that explanation. So how far can I stray from these sizes? Could I put 245/40 (98mm) on the back and 235/40 (94mm) on the front?

No. Use this and it'll tell you the percentage error you will get on your odometer. Anything over 1% is not advisable. And both of your choices will be off considerably.

http://www.miata.net/garage/tirecalc.html

Ok I have a couple of question:
- If I didn't want to buy 17" wheels what would be the optimal tire sizes all round for snow tires on stock 18" wheels?

- Would I actually save money buy buying cheap 17" wheels and then tires, or the perfomance just not as good? If so what would be the optimal size for a 17" tire?

- What is the best snow tire, I might have to drive the Z in the snow and choose to spare no expense (unless this will cost me more than a beater. lol)

If this is too much then just tell me to STFU!

Not too much… in fact good questions. I wouldn’t recommend mounting snow tires on 18” wheels (too expensive and doesn't work as well as 17"), but if you do, this is your best sizes:
Front: 225/45-18” and Rear: 225/50-18”

A better choice for mounting snow tires is 17” rims. Here are the optimum sizes:
Front: 225/50-17” and Rear: 225/55-17”

And, just about any true snow tire is fine. I like Dunlop and Blizzak

--Spike

I think I'm going to go with the 17", will 17x8 all round fit? 35mm offset? I know nothing about tires and actually read up and I just don't get them haha.

Spot on info. I have 225/50 17 fronts and 225/55 17 rears, Dunlop Winter Sport M3's. They're ok so far in snow, but they're no Blizzak. I like the LM 22/25's for mild winters, but nothing beats the WS 50/60's for the deep stuff. And having a RWD 300+hp car means cautious driving is a necessity even with snows.

That will work just fine. Actually, it’s wise to have all the wheels the same size (width) for winter driving. That gives you the option of going “all square” with your winter tires. I wouldn’t recommend “all square” for summer performance tires, but it does work for winter tires. I have 17x8” wheels on all corners. I’ve run the following winter tires setups, and all seem to work OK:

Setup 1) 225/50-17” front and 225/55-17” rear (optimum)
Setup 2) 225/50-17” front and 225/50-17” rear (all square, but works ok)
Setup 3) 225/55-17” front and 225/55-17” rear (again, all square)

Setup 1 is optimal. Setup 2 and 3 are all square allowing rotating tires and/or moving fronts to rear to accommodate uneven wear. You would think Setup 3 shouldn’t work with too tall an overall diameter on the front (especially since I have a Performance model with VDC), but it runs OK.

--Spike