[Jawnathin]

Originally Posted by jdrxb9

I don't think it's that any size has a big gearing advantage - more like the actual time over distance differences are small when you on the limiter with one tire vs another and those small differences are offset by the improved accelerations elsewhere.

Apologies in advance for the wall of text

I think we're on the same page, though maybe we value or weigh the findings a little differently. What I was trying to say is that the acceleration advantage you get from the shorter 235/40 isn't offset by the higher MPH you get from the 245/40. I figured what is better would sway one way or another depending on the course, but even on a course where I thought the 245/40 would have an advantage it ended up as a wash. On a course where you're never on the rev limiter or have to get into 3rd, the 235/40 will be slightly quicker on every acceleration zone than the 245/40 and the 245/40 never gets a chance to 'reel it back in' up top.

The sentiment from some folks around here believe the advantage isn't notable. Sure, I get that it isn't large, but my position is that I'll take any advantage I can get and I would expect every competitive autocrosser to do the same. Using the math from your ProSolo data, extrapolated from a 37~ second course to a 130~ second course (top CS combined time at Nationals) it ends up being a little over a tenth. Lots of trophy, contingencys, and positions have been split by a tenth before and people spend far more to get less than that. I spent $200~ on a single exit exhaust to save 20lbs that I'll never be able to feel or quantify the benefit. I know it won't account for much while out on course, but again I'll take any help I can get

The counter point as I've understood it is that you can exit a corner faster on a 245/40 than a 235/40 due to the additional width and would negate any acceleration advantage you'd get with the shorter tire. But I'm not so sure the 245/40 is going to give you any more usable tire than a 235/40. The benefit of the additional width is a little harder to quantify so I'm not sure we can solve it on this thread, but these are still 9" wide tires on 7" wheels. The larger width will give you 5mm on the outside and 5mm on the inside, but the tires curl up about an inch on each end due to being pinched so contact patch while not loaded up is going to be the same. While cornering you'll certainly lean and use the outside shoulder, the inside shoulder is practically untouched. Even after 300 runs, my inside shoulders looked new. So at the very best, we get an additional 5mm and that assumes the middle/inside of the tire continues to make as much contact as a 235 when rolling onto the shoulder during a hard turn. Similar to my thoughts on the acceleration advantage, I'll take all the help and tire I can get, but what you get here is questionable while the benefit of a shorter tire is quantifiable. Plus as you mentioned, a lower CG and stiffer sidewall. In the end I think there is a advantage to one tire size or another depending on the courses that you run, but there isn't an obvious one size fits all choice here.

Don't look at what I say as disagreement on the findings or observations, but rather acknowledgement that we both understand what is happening and we just happen to approach it a little differently. I appreciate the time you took to look at it. The top two 86s at 2014 Nationals were on 235/40 and 245/40, so we know in practice that they're competitive with each other.

And this may all be moot - if the current RE-71R hype holds up (not to mention the upcoming Rival S), I think the 235/40 ZII (with no contingency) will be really unpopular and almost everyone will be on 245/40-17's anyway

Agreed and the lack of contingency is what swayed me away from moving forward with the Dunlops. I think the compound is fast enough, the Z2 was very competitive with the RS3 v2, and I'd have to hope the Z2* is better than the Z2. My delusions of grandeur has me thinking contingency programs will actually matter for me this year, but I'll take a shot at winning $250 instead of guaranteed $0.

I'm running 245/40 because my hand was forced with the tire I wanted to run. 235/40 wasn't an option, but when I was considering the Z2*, I was truly undecided between the 235/40 and 245/40. The Rival S, Kumho V720, and 200TW R1Rs are all going to be very interesting contenders too. I hope the RE71 lives up to its hype, because I ordered a set of them

Originally Posted by Shark_Bait88

The lack of contingency for the ZII* has me a little disappointed in my choice, as does the trend of everyone "in the know" going with 245/40 RE71Rs. :/ Thought about calling TireRack to switch my order, but I figure I might as well stick it out and see how the Star Specs perform. Given all the data I've been seeing from you guys, it looks like they're very close. My driver mod is going to be a bigger factor anyways.

Side note, anyone planning on running the Wilmington Prosolo in May? This is going to be my first national event and I'm really excited for it!

235/40 Z2* is a great tire and very capable of winning an event, even on a national level. If you still have the opportunity to switch tires, then give them a call if something else catches your eye, but if you can't or don't want to, I wouldn't be disappointed with that choice at all. All of these next gen tires are VERY good and this might be a case of 'grass is greener'.

Congrats on making it to your first national event, it is great to get an opportunity to compare yourself to the best drivers in your region. Signing up for a Pro or NT is a big step forward and you'll have a blast.

Originally Posted by SeanRTR

If you're still working on your driving, and showing up to the first national event, there's really no point in worrying about contingency. Remember all of this tire variance bullshit is about 1-2% of the overall success of the full equation, with driving still being 85+% of the process.


235/40/17 Dunlop ZII's took the fastest time of I think 23 twins at 2014 nationals, so even with newer tires this year, they can't be that slow. I think you'll be fine. The same tires were on my car all winter long, and may be on my car again at the Wilmington pro. I'll see you there.

Great points here. Z2s were excellent tires, the Z2* should be even better. All of these new tires are great, so its not like you're bringing a knife to a gunfight, you're just bringing a different kind of gun.