sonicblue

Questions surrounding this issue are asked a lot, so I thought I'd try to sum up..........


Tire sizes are stated as S/A-D: S=section width (in mm), A=aspect (as a %) and D=diameter. (in inches) For example, a 205/65-15 tire is 205mm wide, the sidewall is 65% of the section width and it should be mounted on a 15" wheel.

For rolling diameter (RD), here's the calculation: RD = S * A / 25.4 * 2 + D

So, my 3.2TL stock tire (as above), has an RD ~ 25.5 inches. If I want to upsize my wheel, I need to aim for a wheel/tire combination that comes as close to this RD as possible. Since people more often that not select their new D first, let's assume it's the S and A we need to figure out.

Rearrange the equation above and get: S * A = (RD - D) / 2 * 25.4

Plugging in my stock RD from above, gives me: S * A ~ 108. This says that, for a given 17" tire, if S * A ~ 108, I'll have the same RD (and ride height) as my stock tire. Incidentally, what you're basically doing is calculating the revised needed sidewall height after accounting for the increased wheel diameter. Also, your answer here is in millimeters. Thus, we're looking - in this case - for a tire with a 108mm sidewall.

So, take some 17" tire size - say, 215/50-17 - and calculate S*A. In this case, that's 215 * .50 = 107.5. Pretty durn close, so this is a great fit. Others may come close, too: a 235/45-17 tire has a sidewall of 235 * .45 ~ 106. This is close but about 2mm too small. This would translate to 2mm less ground clearance and a 2mm increase in fender gap (not much, but off when compared to the 215/50-17).

To find out how your speedo will be affected if your RD does not match up, you need to calculate revised RD / stock RD. Say you decide, screw it, I got a buddy giving away a set of 225/45-18 tires and you'll make 'em fit (stock as above). Revised RD is ~ 26.0 inches. Calculate 26.0 / 25.5 ~ 1.02. This means I'm about 2% too big, and 60mph on my speedo is actually (60 * 1.02) = 61.2mph. This is because the speedo measures axle revolutions and assume one revolution carries me a certain distance. If my RD is bigger than stock, one revolution takes me farther than before so my actual speed must be higher than expected (and vice versa).

NOTE: none of this discussion considers clearance issues on any particular cars. This can be applied to upsizing tires on any car, with clearance issues brought in as a last step.

Hope this helps.

zoobobs

this is an awesome post. thanks.

TLBaller

you lost me at the RD lol fuck it, ill just stick to what i know - 2.5TL can fit 18x7.5 with a 40+ offset

sonicblue

:lol: but, you still need my post. Although you know what rim fits on your car and what offset, this thread is about telling you what tires you can wrap those in to get the best match to OEM

sonicblue

some more info I got from another forum:

just for reference, a 7.5" wide wheel and a 215 tire will have mostly a flat sidewall profile in relation to the wheel. If 215s are the only tires that will fit under your fenders, then I would stick with a 7 or 7.5" wheel. you're less likely to curb the wheels with a 7" and 215's though.

Since my stock wheels are 15x7 with a 205 tire, I'm pretty sure I can fit a 215 tire up under there. And, since I don't like the bloated tire look at all, I'm shooting for a 7.5" wheel. The above poster makes a good point, though - if I put the 215s on a 7" wheel, the rubber would stick out a little more and you'd be less likely to curb the wheel itself.

jerseydriver

your not kidding when you say your from collegeville.

jk your one smart dude because that right there is a mouthful.
i will have to read this post 40 times and slow to get it.

as for offset, i dont know what offset i have. i have 225-40-18's on my 3.2
so hold up why i do this math

AjaxVCU

Awesome Post, that is a huge help. Just what I was looking for!

Danoli

theres a much quicker way

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

this tire calculator will give you all the same info but without all the fancy math.

enjoy

sonicblue

Fair enough, I'm just not a fan of black box math. That calculator has you just punch in some numbers. I did this for anyone who likes to actually know what the numbers mean and - more importantly - how you can vary them to get to a desired end.