School me, how is 225 on a 7in "Stretched"?

[Earendil]

I've spent the last week reading more then I ever thought I would about wheels and tires. And one answer still eludes me.

The common suggestion for a 15x7 wheel is a 205/50 (or 205/something).
It would seem about 80%-90% of people suggest this. A few times you catch the 225/something suggestion. Even more rare is the discussion on why one would choose one over the other, except to say that a 225 is better suited for an 8" or 9".

Now, I can do math. 205mm = 8.07in. And 225 = 8.8in. So this all was making sense to me. That is, until I remembered that I'm running 205/60/14 on my Basketweaves, which are supposed to be 14x6.5. And now I'm confused

I know that 195 is the suggested stock tire, but 205 fits the basket weaves perfectly. They don't even appear to exceed the lip, and if they do it's only 1/4. A far cry from some of the pictures I see posted here where it looks like some people have 1" hanging over the rim

So, if a 205 fits a 6.5", how does adding 0.7" to the tire making it a 225 not fit a 7" ? Shouldn't this only be an extra 0.1" per side?

School me. Please fill in my gaps in knowledge :(

[Wh33lhop]

Section width is not tread width. The higher aspect ratio you go, the less tread width you actually have typically. So the 205/60 will have a nominal 205mm section width (at the maximum width of the "bulge" in the tire sidewall) but will have less actual tread width than, say, a 205/50 since the sidewall has less bulge in it. Additionally, less sidewall means less ability to absorb the width difference between the tread and the wheel.

[nrubenstein]

OK, first and foremost, 225/45 on a 7" wheel isn't stretched. It's a bit squeezed. 225 will be just fine on a 7" wheel, it's just not quite the optimal wheel width for the tire. I mean, hell, I've squeezed a 275/35 hoho onto an 8" wheel. There's a lot of flexibility.

Secondly, the taller the sidewall, the "wider" the tire you might run on it under optimal conditions. For example, my truck comes with 16x6.5" wheels that have a 245/75 tire mounted. People run 265s on those wheels without issues. Basically, what it comes down to is that the shorter the sidewall, the closer the tire width should match the wheel width. Otherwise, you're losing a lot of the benefit of control that comes with the shorter sidewall. (Remember those 275/35 hohos I ran on an 8" rim? They were pretty sloppy.)

[Earendil]

Thank you gentlemen. That not only clears it up, but it also makes sense in my head.

So if my understanding is correct, than you really have to take into consideration the height of the sidewall when figuring out if a tire will fit.

If I may extend my original question, since you two seem to know your stuff, why would a person go with anything but the optimum tire size for the wheel? My intuition leads me to believe that given a set wheel width, that a squeezed tire would provide a larger contact patch while sacrificing turn in. Where as the optimum tire wouldn't float and provide good turn in, but not have the largest contact patch possible.

If the previous intuition is correct, would I be correct in guessing that if one wanted to maximize the lateral G that could be archived on a skid pad, a squeezed tire is likely to be better. If you wanted solid rapid turn changes like in a slalom, that the optimum tire might be best. And somewhere in the practical world of street and track, there is a balance that needs to be struck?

Or are squeezed tires purely a result of putting whatever tire is available on whatever wheel is available?

[KennyT]

First off, at equal vehicle weights, tire construction, and psi contact patch is the same. What changes is how fast the tire warms up while driving.

A slight stretch is frequently used in racing to stiffen the sidewalls under cornering loads.

What you should do:
Find a tire you like in terms of cost/performance/characteristics, then find the width that best matches what you will be doing. For example, if you are running autocross in a barely modified car you should use a 205 width tire. On the other hand, if you have a lot of suspension work and power you should run wider due to the heat generated each run.

After you find your ideal tire and width, match your sidewall height to either oem (so your speedo reads correct), or taller/shorter to change your final drive ratio. Think of Miata's on 13'' wheels vs 15''.

If you are going for performance tires come before wheels, if you are going for looks then my post just wasted your time.

[Earendil]

First off, at equal vehicle weights, tire construction, and psi contact patch is the same. What changes is how fast the tire warms up while driving.

A slight stretch is frequently used in racing to stiffen the sidewalls under cornering loads.

What you should do:
Find a tire you like in terms of cost/performance/characteristics, then find the width that best matches what you will be doing. For example, if you are running autocross in a barely modified car you should use a 205 width tire. On the other hand, if you have a lot of suspension work and power you should run wider due to the heat generated each run.

After you find your ideal tire and width, match your sidewall height to either oem (so your speedo reads correct), or taller/shorter to change your final drive ratio. Think of Miata's on 13'' wheels vs 15''.

If you are going for performance tires come before wheels,

Quite helpful indeed. My former DD 325i is pushing towards track rat status. I want a tire that will be suitable for HPDS, autox and weekend on country roads. Looks like 205/50 will be right up my ally. I'm just one of those pesky fellows that has this rather strong desire to understand, not just do what is common or what he's told. So thanks for the help!

if you are going for looks then my post just wasted your time.

Looks be damned :)

[KennyT]

AutoX and HDPE are completely different animals when it comes to tires. In autoX you want the bare minimum in width in order to get them warm as soon as possible considering you get maybe 60 seconds on course for every 10 minutes of the event.

HDPE you are on track for several minutes at a time and you will overheat an autoX grade tire in 3 minutes of track time.

Assuming you are aiming at a RE11/Star Spec/etc tire you should aim for 205/50/R15 for autoX, if you are going for HDPE I would go 225, maybe larger if you have a lot of suspension and engine work.