THSCC Forums

Thanks for the input on the street tires! I would be using the stock 8 inch wide wheels and would want to use them as "daily driver" tires mostly. Therefore that currently limits me to either the Conti DW or the Michelin PSS to get the tall oem diameter. I'm not inclined to seriously autocross a Mustang in general, let alone an automatic. If I "really" want to drive the Mustang to autocrosses it would make more sense to put a hitch on it and tow the CM car . . .

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Dick Rasmussen

FS 50 2018 Mustang GT

Spot on. I was trying to find a good link to post for this since I have read some good data on the subject in the past, but much of the evidence is anecdotal making the good data difficult to find in a search. Also, much of the discussion centers around the 205/50/15 and 225/45/15 tire size and if one should purchase 8 or 9 inch wheels. For people trying to decide what size tire to put on a stock wheel or an after market wheel they already have, this is like putting the cart before the horse.

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92 Miata 1.9L naturally aspirated. SSM 21

Thanks James, I'm going to check out that site/username.

The old engineer in me, emphasis on old, worries about making conclusions based on one particular street tire. In other words, what "works" best in autocross use for say a Hankook RS3 in terms of wheel width to tread (or section) width might not just be the best combo with a Rival or StarSpec or, etc...

I'd really like to see some instrumented and carefully controlled tests for skidpad (steady state grip) and then an autocross using different wheel widths and tire widths in all combinations.

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Current stable:
2019 BMW M2 Competition slicktop 6MT
2011 BMW M3 sedan slicktop 6MT
2007 BMW 328i wagon (slushbox for now)
1975 CanAm 125MX2

Yes, I think I agree with your implication Chuck. See the thing with the whole "Miata's love a 225/45/15 on a 15x9" claim is based solely on the RS3 since it was the only tire of that size (other than maybe the R1R but I'm not sure which came first) at the time that this became a big deal. As a lot of us know, the RS3 has much softer sidewalls than a lot of other EPST so running a square, if not slightly stretched setup makes sense.

And with Andy Hollis, he is making the claim based on ST(S) requirements of 225mm or less tire sizes on wheels of 7.5" or less. The R1R is the tire of choice given these requirements, which also has relatively soft sidewalls so running the pinched 225 on a 7.5" would result in very sloppy steering response.

That being said, if the star spec came in a very odd size, like 235/40/15 or 245/40/15, I would bet it would do fine on a x9" wheel relative to the RS3 given the significantly stiffer sidewalls.

I have never heard of anyone doing an R1R 225/45/15 on a 15x9" since the magical R1R is the 195/50/15.

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2011/2012 Autox VP
2013/2014.5 President
2013 Top Gun

2015 Fit

22R-EC => 4G63 => D16Y7 + D16Y8 => EJ255 + K24Z2 => K20Z3 + K24Z2 => K24Z2 + M54 => L15B

Dick,

From having both tires (I have the PSS on the M5 and the DW on the M3 for daily driving), the PSS is a vastly superior tire in two ways: dry grip is much better -- in the Tire Rack/C&D tests last year, the PSS was 0.2 behind the AD-08 in dry autocross and well ahead of the Star Spec while the DW was 1.9 seconds slower (~60 sec course) than the PSS.

The other problem I have with the Contis is uniformity. They only have about 4k miles on them, and I've been having shaking at speed issues. Had Jeb roadforce balance them carefully, but I still have issues. One of the tires showed 26lbs of roadforce and even after changing its position on the wheel based on the machine's indication, it didn't change. The other one went from 24lbs to about 16lbs...still too high.

Continential's uniformity control in manufacturing is child's play compared to Michelin. The Pilot Super Sports were all less than 5lbs roadforce as mounted with one of them 0. My son has the DWS on his S4, and he has had on-going balance issues showing up (and need attention again now which of course is now my problem since I bought the car from him as he moved to Silicon Valley for a new job).

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Current stable:
2019 BMW M2 Competition slicktop 6MT
2011 BMW M3 sedan slicktop 6MT
2007 BMW 328i wagon (slushbox for now)
1975 CanAm 125MX2

A lot of the work was also done with the 205 tires on a 8" wheel since 225s on a stock power Miata is slower on most tracks. The real question is does what works on Miata size tires apply to Mustang/M3 size tires.

No doubt. I would imagine that just the rolling resistance and increase aero drag of the 225 vs the 205 could make some difference in a low powered car like a stock Miata on track. What would be nice is to see data on section times, corner entry to exit, for all corners on track.

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Current stable:
2019 BMW M2 Competition slicktop 6MT
2011 BMW M3 sedan slicktop 6MT
2007 BMW 328i wagon (slushbox for now)
1975 CanAm 125MX2

^^^ - There are telemetry/datalogs; however, people tend to be tight lipped around what their data shows if it can provide an advantage to a competitor.



I linked/quoted a write-up of the Rivals done by Emilio on page 6 of this thread, dude is smart.

Being in Miata land, having no power, and 15" tires not having the biggest selection, this all needs to be taken with a grain of salt. The proof of what wheel/tire size combos work comes down to NASA TT* or PT* racing and laptimes. The other consideration is what does your class allow you to run, so if you were limited to an 8" wheel, then a 205/50 would be the fastest option available, whereas if you could run a 9" wheel, then look at 225/45 tires.

What James Short said is extremely true regarding tire sizes (or rather the lack thereof) because there are a bunch of people that would be ecstatic to get their hands on a 245/40R15 if somebody made them. It literally goes from 225 to 275 in 15" sizes.

More Rival news from Emilio @ 949racing

The secret weapon is the cat pee smell that overcame the competition. Seriously though, they stink when hot.

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92 Miata 1.9L naturally aspirated. SSM 21

I decided to go with 255/40/17 Rivals for the STR S2000. The 245s were shorter, and since the car is already gearing challenged (top 2d gear is 58mph) vs the AP1 I didn't want to lose any more speed. When/if I get Hondata and an exhaust maybe the power and increased rev limit will overcome it and 245s will be the better choice, but until then....

I will say that running 24540/17 Z1* on the front for the past few events made the turn in super crisp.

Being a cat owner, I suspect i will be quite at home with the Rival's unique smell

edit: not a tire or petrochemical engineer, but it appears (via an uber-quick Google search) that urea compounds are used to stabilize vulcanized rubber compounds...so maybe this has to do with the better wear characterstics observed by Chuck, Jordan and others? <shrug>

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Steve Carter
1972 Datsun 240Z-- resto pics at http://picasaweb.google.com/srcartermd
2007 GPW Honda S2000-- STR 86

Chuck,

Thank you VERY much for this information. Either of these tires would be primarily a street tire so being "civilized" is rather important.

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Dick Rasmussen

FS 50 2018 Mustang GT