RoboX

I found this great site the will calculate the correct tire size for you.
Hope it helps you

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

CIONIDE

iTrader: (1)

That one's good, but this one gives even more info regarding fit:

http://www.rims-n-tires.com/rt_specs.jsp?postId=906

Keep in mind that the stock tires are 205/55x16" with +45 offset

RoboX

oh cool thanks

CIONIDE

iTrader: (1)

Here to help!

swartzautoman

iTrader: (3)

yep, the second one is the one i always use

ScionicXTC

Also check out the chart here.

MikeInABox

Wow that second site interactive thing is great.

One rule of thumb I always heard to keep the overall diameter of the wheel/tire the same as stock is when you go up an inch on wheel size, go down 10 on the sidewall. Like if you have a 16" wheel with 50's, go 40 with a 17" wheel, 30 with an 18" wheel, etc. Not sure how accurate that is, but I've read it more than once. Obviously you could only do that to a certain extent or you'd have almost no tire left.

jo_maoma

problem is the "50" in a fifty series tire is the ratio of the width not the size of the side wall.

if you use this rule of thumb with the stock 205/55/16 you end up with a tire 2 inchs smaller
by the time you get a 205/25/19.

RoboX

eh my wheels and tires are .5" bigger then stock. i just like the way they fill the wheel wells better. my speedo reads slower by about 3mph. haha itll also read less in my millage aka longer warrenty ;)

MikeInABox

Yeah I'm not saying it's right or wrong. Just something I've read.

CIONIDE

iTrader: (1)

I'll say it... It's wrong.

Overall tire and wheel diameter is a formula:
(tire width) x (aspect ratio) x (2) + (wheel diameter).
Save yourself the headache, and see all the other affected parameters,
by using the tire calculator/comparer I have in the 2nd posting of this thread.

That "rule of thumb" going down in aspect ratio by 10 for every 1" increase in wheel diameter is useless.
I'm not trying to be mean or rude MikeInABox, you know I enjoy posting with you;
I just don't want anybody to try it, and then be disappointed.

MikeInABox

Yeah no worries man. I might have what you change per inch wrong, but I swear there was something similar I read that gave a simple way to keep the overall diameter close to the same as stock when changing wheel size. It was in a magazine I had but that was several years ago so there's no way I'll ever find it. Maybe it was going down 5 instead of 10. But my tire size-o-meter needs recalibrated because my last wheels the rears were 18x10 with 255/35.

In any case, it's best to use that wheel/tire calculator and see what others have done to decide what's best.

CIONIDE

iTrader: (1)

Mike, I played a little on the calculator: if you keep the tire width the same as stock (205 in our case), then if you drop aspect ratio 5% for each additional 1" in wheel diameter, the overall diameter stays reasonably close to stock.

The problem is that almost nobody is going to go up in wheel diameter, and keep the same tire width as stock. In any case, as you said, "best to use that wheel/tire calculator".