I'm considering the new wheel/tire route. Will probably go with 18s due to the very high cost of 19s.

What specs should I look for so the speedo will still function properly?

Then again, what would need to be done to have it restandardized?

I know many of you have aftermarket wheels, so I'm sure you've been down this road.

Thanks.

 

Get out your calculator and do the math.

stock is 215/50-17, right?

.50 * 215 = 107.5 that is your sidewall height

107.5 * 2 = 215 (your two sidewalls)
215/25.4 = 8.5" (convert metric to inches)
8.5 + 17" = 25.5" that is your target height for any wheel/tire combo

 

the stock rolling diameter is 25.46" so 235/35/19 is the most appropriate (overall diameter is 25.47"). getting this tire size means getting a 19x8 wheel since the tire width is most appropriate for 8" wide rims. and since you are getting 8" wide rims, you will need at least a +50mm offset for the least amount of rubbing (you probably won't) when lowered. now if you decide to get 7.5" wide rims, you can go slightly lower on the offset as the rim is not as wide...or conversely, get a even higher offset if you go with 8.5" wide rims. but you are better off getting ths 8s. simple huh???

 

Really it's tires that make the most difference with speedometer problems.

For 18s it's 225/40/18
For 192 it's 235/35/19

Keep in mind 225/40/18 is not a perfect match, 225/45/18 is better. Only problem is there aren't a lot of wheels to choose from.

For offset:
18s
18x7.5 +45
18x8 +50
18x8.5 - don't bother, they won't fit right.

 

check here:

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

 

Thanks guys. You've cleared this up quite well for me. Thanks for the calcs too. I always wondered how the ride height of the tire was actually calculated.

Now I have to decide on 19 vs. 18........

 

BTW, after changing your wheel/tire setup, did you have to get your speedo re-calibrated? With the plethora of available combinations and the amount of changes each will contribute, I'd like to think that I can count on an accurate speedo regardless of what combination I wish to go with.

Also, rubbing is a HUGE problem for me. Should I totally forget about 19's and just focus on a set of 18x8 or 18x7.5's? I plan to do the spring thing in the future but my list of future mods is getting longer than my time allotment. I'd like to go with the easy mods and deal with the other ones later.

I figure my current stereo mod is going to take me at least another month to work out. I'd like to keep a wheel/tire mod in the back of my head though!

 

I gather you are going for cosmetics over any kind of real performance gains. You realize of course that large diameter wheels kill horsepower, don't you? 18s will definitely provide less power loss than cartoonish 19s.

rw

 

It's more compicated than that if you want to know the exact size. Of which I recall there are only 1 or 2 other tires that are the sames as the 215/50/17 Michelins that are on the TLS.

The RPM on the Michelins is 819.

What you need to do is compare the RPM rating provided by the various tire manufactures. I suggest that you stay within the 825-815 range if you want to stay really close.

The Yokohama AVS ES100 215/50/17s are 815. Also the Falken Zeix 512 215/50/17s come in at 821. I think there is a Dunlop and Kumho that are pretty close too.

For what it's worth, I went with 225/45/17 Toyo T1-S's and they were 828RPM. I've had no problems with the speedo and checked it against two different radar devices and no difference was noted. By calculation it's less than a 1MPH at 60.

So you really have a lot more choices if you expand the RPM range your willing to be a bit flexible.

Also, don't forget to look at all aspects of the tires. Sizes are only a general statement. No two sizes are the same, meaning that a 215/50/17 from Michelin does not have the exact same tread width, section width, diameter, weight etc as the 215/50/17 from Yokohama.

Also, tread wear is not comparable between various tire manufactures. It's primarily a guide, and is really only accurate for comparing tires within the same manufacture.

Lastly, don't discount weight as a key issue. Lighter tires make for faster cars. And reducing the diameter size a bit (increasing the RPM) also helps lowend take-offs.

Just look at what getting the best combination of tire specs and wheels did to my 0-60 and 1/4 mile times.

0-60 6.19 secs
1/4 14.659 secs - and I have no other mods.

So do your homework.

Ruf