Tire size question - OK, so call me stupid ;-)

[Ed Kellock]

205 is the width from sidewall bulge to sidewall bulge
50 is the ratio of that width to the heighth of the sidewall, called
"aspect ratio"
17 is the diameter of the wheel

To vary with the same diameter wheel means you'll be changing overall
diameter of the tire.

Vary just the width: 215/50-17 will be taller and cause the speedo to read high

To vary both the width and aspect ratio means you'd be going
significantly wider or narrower which might cause fitment issues
relevant to the wheel width.
225/45-17 would be just a tad less in overall diameter and will cause
the speedo to read a bit low, but not by more than 1%. (BTW, this is a
fairly common size).

Check Tire Rack.com for a more technical treatment.

Tire Rack also show specs on the recommended wheel width for a
particular size tire.  You have to search for the tire size, pick one
and go into specs to see this.

There are a few good tire size comparison utilities out there.  I have
a favorite and can send it if you like.


HTH,
Ed


On 11/9/06, Tom Winter <tom at freeskier.com> wrote:
> This is so basic that I'm almost embarrassed to ask, but I'm going to do so
> anyhow.
>
> My wife's BMW needs new tires, and the size of her current (almost bald)
> rubber is 205/50/17.
>
> Ok, so what exactly do these numbers mean? How do I know how to sub a
> different sized tire on the rim, and if it will fit?
>
> Which measurements are important, which can be different? (within reason).
>
> Because she will be selling the car soon, I want to put some decent used
> rubber on the car for the 1-2 months she'll be driving it. As an aside,
> hopefully I can convince her to ditch the BMW brand, but although she likes
> my Audi, she's pretty stuck on BMW
>
> MAC: Yeah, I know. I should know this stuff, but I've always run stock rims
> and the appropriate sized tire on my Audis, so I've never bothered to learn
> anything. At least, unlike my wife, I can change my own oil and fix a flat!
> LOL!
>
> Thanks in advance for cluing me in to this esoteric knowledge.
>
> Tom
>
> '95 S6 Avant
>
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