245 or 255 tire on a 9" wide wheel?

[jledford]

I'm debating on what size tire to put under the M3 for the season. I have a set of 17x9" wheels coming in soon and it's about time to order tires. I plan on sticking with Hankook RS3s for the first part of the season with a possible switch to the Rivals once those are released. The RS3's are literally $8 more for the set of 255/40s versus 245/40s. That's $8 total, not per tire. I want to stick with the RS3s over the ZII right now for money savings and track day use.

I've talked with the wheel supplier and I should be able to fit 255 tires with my current modifications. This will be a square setup on the car, no stagger. However I've read some articles where certain cars were faster with a 245 on a 9" wheel versus a 255 on the same wheel. Then on the other hand the guy that won STU at Nationals last year was running 255s on an M3. Perhaps the taller 255 tire helps with the gearing some as well.

So what sayeth the ALSCCA crew, 245/40s or 255/40s on a 17x9" wheel?

[kilgojr]

going by Tire Rack, a 245 on a 8.5 gets you 1/2 inch more tread then a 235 on a 8. They don't show Tread Width of a 255, but the 255 on a 9 gets you 1/2 inch more Section Width then the 245....this is inline with the 245 from the 235. In Autocross, the more rubber you can get under the car, the better the results (in my experience, even at the cost of more weight on each corner). I know Sean has been running 255's on his M3, ofcourse he has double the horsepower you do, but the car is very well balanced. I think he wants more tire underneath his car, which would be good when you have 400hp.

So for my 2 cents, I'd order the 255's.

[nicktcr]

Bigger is better. Plus, when you get those halfway used up and are tired of them, that's the size I like to run on my 240

[Cholewa]

I ran 245's on a 7'' wheel and it was meh.

A tiny pinch of stretch is better than a swath of tread you can't even use. You can always change the sidewall size to enhance (or suckify) gearing.

This goes great with my thread asking where the benefit of tire size drops off. It's too bad we never got a solid answer.


Get out a calculator and figure out your contact patch for each, the overall diameter (to determine any gearing change) and weight differences. That data should help you determine which one is best for you.

[rodhx]

In this case you likely want the 255s if they fit in the wheel wells. Then we need to find a day for me to come to Auburn and test fit your wheels/tires on my car

On the other hand, regarding the RS3s it has been shown that the 225/45-15s are at their best when slightly stretched on 9" wheels versus 8" wheels. I wonder if that means for that particular tire you would be better off with 245s???

I will be staying on 245s for a while. That's what is on my car and I had no fitment issues during sunday's test-n-tune.

[bimmertech]

What Rod said. I was just too lazy to type it all out.

[e30Racer]

Go with the 255's. Here's what's likely to happen if you go with the 245s....

You like the 245s in the RS3 and they handle well. Then the Rivals come out and you're thinking about getting them. Then you're back to the question of...245s or 255s? The RS3 245s are nice, but you could use a little more grip. So you say you'll go with the 255s in the Rivals, but then Rivals are too expensive and then you say well I'll stick with the RS3s, but I am going to try to the 255s next time.

The moral of the story is that in the end, you're probably going to end up getting 255's in either RS3s or Rivals down the road, so why not just go ahead and get the RS3s in 255s now.

Another way to look at it, similar to what Rod said, whatever your constant is (17x9 wheels in your case Jerry, 225 tires in Rods case) you want to fit it with the biggest variable you can. If you have 9in wide rims, fit it with the bigger set of tires (255s). If you have a 225 tire, fit it with the bigger rim, the 9in rim.

If nothing else always remember....Bigger is Better (in moderation)

[HPEVOX]

255 RS3 should be square on a 9" wheel. My 265s are square on 9.5" wheel as you saw Saturday. And remember Jerry


Mine = 1" bigger diameter... and .5" wider

[e30Racer]

255 RS3 should be square on a 9" wheel. My 265s are square on 9.5" wheel as you saw Saturday. And remember Jerry


Mine = 1" bigger diameter... and .5" wider

What exactly do you mean by "square"?

[HPEVOX]

What exactly do you mean by "square"?

The sidewall is near perfect perpendicular to the ground when 265 is on 9.5" wheel. this >> Square = l__l .. not this herrafrush = \__/ nor this /__\ racecar

[e30Racer]

do you think that square sidewalls are best?

[HPEVOX]

I know his main concern was with fender clearance the other day. Just saying how it would fit on the wheel and how it would/wouldn't tuck under the fender. I'm sure he prefers 255s (guessing), but seemed like it was more concern with tucking in or not.

I have no insight on BMW gearing

[Cholewa]

A pinch of stretch is best, the more sidewall the more squirm and the less crisp of turn in.

[JohnW8]

In my "testing" I've found that a stretched tire requires more air pressure to maintain a good contact patch, whereas a "square" sidewall allows for more adjustability of pressures without caving in the center of the contact patch. With a pinched sidewall you need higher pressures to get rid of the wallowing on the sidewall.

[nicktcr]

It depends on the kind of tire being used. The cheaper and less "sporty" the tire, the more benefit you MAY see from some stretching on the wheel. If the sidewalls are inherently soft and squishy, running a slight amount a stretch (along with the added air required) you can get rid of a little sidewall flex. But as you get more into tires designed to have a less comfy sidewall that is designed not to flex, but rather withstand greater lateral loads, you will see less and less of any previous advantage, and will only see benefit from simply fitting more rubber underneath the car.

Basically, stretch the all-seasons to make them less crap (still crap), and pinch the r-comps/slicks all you want, because those sidewalls aren't giving regardless of air pressure.

There will always be a point of diminishing return where going bigger will either no longer achieve greater results or may start being slower (usually not the case with tires). Finding that balance really just takes either experience, testing, or some combination of both. For that kind of tire, I really see no reason not to go wider unless either the rules do not allow, or if you're going to have fitment issues that are not allowed to be addressed within the rules (i.e. fender rolling/pulling isn't allowed in that class). I think more often than not, the more rubber you can get under the car (again to the point of diminishing return) the better it will be.

Also, a 245 vs. 255 "stretch-wise" on a 9" is almost not noticeable at all. On a wheel that size you'll need to drop down to at LEAST a 235 before you'll be able to tell there's any stretch whatsoever. I ran a 225 on mine for a while, and it was just slightly stretched.

Go bigger!