Well...I don't claim to be the "guru"...or I wouldn't have posted asking where one was...but...here is what I've read/learned/heard. Some have already run the 245/275 combination on their stock wheels...but they were the Track model. This gives them the extra half inch on both fronts and rears to work with. I don't see any problem running 245's up front even on our Performance wheels. But, I think 275's are going to have an unsightly buldge and wear poorly on the rears. 265's are the largest I would run on rear. Now I haven't searched for tires in the following sizes, but I believe the following provides the best comparison ride: 245/40/18 up front and 265/40/18 on rear. The front would be under stock by .3" and the rears would be under by .4". This is only off by .015 and is completely acceptable. To address a question above, many tire shops use a rule of thumb of 2% acceptable variation, and some even use 3%! Obviously the objective is to come as close as possible with the width you want to run. The .4" variance on the rears would provide a theoretical error of +1mph at 65mph. Remember that most performance tires even have a minimum usable tread of .25", which means when your stock tires are bald as posted above, your stock diameter is around 26.2" rather than where they started at 26.7" and the replacement tires are 26.3". So for the first couple thousand miles you are actually closer to stock spec. than the worn ones you're running on currently. It's that close. The speedo cannot detect this small variation accurately and unless you are doing 100mph, you are unlikely to even notice!

That's my take on the whole thing....feel free to argue with me if you are no more educated than myself...otherwise, I'm open to a free education if your smarter.

 

That's good data, thanks. Did you mean that we'll have a .4 variance with the 265's?

The following calculator confirms that and also indicates that there is no variance in diameter with the 275's. Check it...is it right? http://www.miata.net/garage/tirecalc.html

This place has the recommended "plus one" size as 275 in the rear. http://www.discounttiredirect.com/

I'm still confused

 

Driven - Thanks for the input, it sounds right on. I've been using that calculator for every combination possible, and the 245/40 and 265/40 looks pretty good. Unfortunately, there's not a 265/40/18 size available in the tire I want, which is the Potenza S-03 Pole Position. Even if I wanted another tire, there's only a couple of options in that size, none of which I can find are very desirable. So, my solution I think is going to be 235/40 and 255/40. I'll be losing .6" diameter in the front, and .7" diameter in the rear. But this will maintain the front-rear ratio with only a .1" change.

BFG10 - You are as confused as I am, or was. But here's the thing with the 275s. They are meant for wheels with a minimum of a 9" width, and the perf model is 8.5" I believe. It seems to be the consensus here that it could cause a bulge and bad wear from rolling. I almost went with the 245/40 and 275/40 combo, until some guys here changed my mind. See my size decision above, and see what you think.

 

The Performance model wheels are 18x8 in front and rear. Only the Track (Ray's) are 18x8.5 in the rear.

 

I agree. I want the S-03's too. There is nothing in a 265 that's any good. I'm not sure about the 255/40 though. Might work. But how would it look? Will there be more gap between tire and rim? Will it look too small? Is it a nicer lower profile look? All that...And how far off will the speedo be? I'm not sure I understand the front/rear ratio issue.

I keep thinking that maybe I'll just stick with stock sizes since I know how it fits and what it looks like and I'll get the S-03s. I will have accomplished half of my mission. To replace the crappy RE040s. As far as going with a wider tire, that might have to wait.

But if you do the 255's let me know.

 

I think it would look pretty good. You'll only have a .6" and .7" difference in diameter, which really is not going to be noticable I don't think. So that's a .3" and .35" more of a gap between the tire and the top of the wheel well (I'm guessing that's what you meant, not between tire and rim). Also, the sidewall will be a little smaller, and with the fatter tire, you'll get a nice low profile look. I've done all my calculations here:

tire size calculator

According to that, it will make my speedo off by a little over 2%. So when I'm going 60, I'm really only going 58.5-59, which really doesn't seem like a big deal to me. Might save me from a ticket, who knows

As for the ratio thing, I was just thinking that the rear is a bigger diameter than the front. So if I don't make the same diameter change in the front as I do the rear, it will make the car a little higher on one end than the stock setup. Probably not a big deal, but I'm not sure. Well, I just ordered my tires, so I'll let you know how they look!

 

I guess you went with the S-03's right? Tirerack?

Definitely let me know how they look and fit!

Maybe even a few pics if possible.

 

Yup got the S-03's. I don't feel like going through the hassle of shipping and taking my tires to an installer, so I didn't order from Tirerack. I went through NTB, and they actually matched tirerack prices, plus $15 per tire, not bad. On top of that, for 10% a tire I get the hazard protection, which you wouldn't get from tirerack. They should be in next week, so I'll let you know, with pics. Later.

 

Well...as mentioned...I didn't shop for those specific sizes. Do Toyo's come in 265/40/18's? I here they are a very good tire. Anyway it seems everyone above kinda has everything worked out. One correction, however...the Performance does not come with 8" wide wheels front and back. The front is a 7.5" and the rear is a 8". The Track has a 8" front and a 8.5" rear. Those of us with Performance model have the same damn narrow wheels that are on the base model! (Just different styling and diameter)

 

Yeah I'm glad I found the specs for the wheels before I ordered, or I would have considered a fatter tire. I think I'll be pushing it as it is, because the 235/40 fits a minimum of a 8" rim, and the 255/40 fits a minimum of a 8.5" rim. So I may have a slight bulge, but I'm not too worried. I think a 265 or 275 would give too much roll and bulge. Damn I wish we had wider rims!

 

infinite z,

Did you get the tires yet? I'm anxious to hear how they perform, fit and how they look. Let us know when you get them rolling.

 

Nope they haven't come in yet. I'll let you know if they ever get here