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Everything posted by Kip

  1. According to Michelin, the 245/35r18 will fit an 8" wheel. That said, it creates a f/r diameter ratio of .95. That's a little bit below the stock range ratio for winter of .97. I was assuming that since the stock sizes won't throw errors, anything within the range of ratios from the stock sizes would also not throw errors. This one, being outside of the range may throw errors. I wasn't willing to risk it. I bought the 235/40r18. If you go with the 245/35r18, please let us/me know if they fit and throw errors. -Brad
  2. Updated after initial posting... Yep. As predicted, I made a mistake. The stock size for the Carrera is 225/40 r18. I entered it as 225/30. Updating to the proper spec changed the low end of the stock f/r tire diameter ratio range. This bumped one of the original options off the list. I've updated the long, rambling detail below to reflect this. I've also updated the attached .pdf with my data So I nerded out last night and did some analysis to answer my own question. Thought I would share it with the group in case anyone else is considering the Pilot Sport 4S for your 996 C2. If you don't want all the background info, jump down to "Results". My situation: I'm shopping for a daily driver/summer tire for my 2004 996 40th Anniversary model. It has the VF Engineering supercharger. It is running 8x18and 10x18 wheels. My goal was to find a good performance tire that performed in the wet and dry, might be able to handle a track day or two, isn't excessively loud, and would last for ~15k miles at a minimum. I liked the Pilot Sport 4S. This tire is made in the proper front tire size (235/40 r18), but not the proper rear size (295/30 r18). Michelin does make a 285/35 r18. I wanted some level of assurance that I could choose a front tire that would work with a 285/35 r18 without causing problems for PSM or ABS. I'm not too concerned about speedometer or odometer inaccuracy that can happen with tire size changes. Base assumption: My understanding is that the speed sensors that "tell" the ECU to throw errors to PSM and ABS are sensitive to changes in tire size. Specifically, changes in tire size differences between the front and rear tire diameters. So if a car has all the same size tires, it has a 1.00 ratio for front to rear tire diameter. With this 1.00 ratio, the ECU interprets everything is fine as long all four wheels are turning at the same speed. If you were to put a smaller tire on the front only or a larger tire on the back only, you would deviate from the 1.00 f/r tire diameter ratio. The speed sensors would see the front tires turning faster than the rears. If this deviates beyond whatever the ECU deems as acceptable, it will throw PSM and ABS errors. Based on this, my decision for a front tire to match the 285/35 r18 needs to be directed by the f/r tire diameter ratio. Analysis approach: Using Michelin.com, TireSize.com, and TireRack.com, I collected info about tire diameter, width, and sidewall for three stock sizes for a 2004 996: Carrera, 40th Anniversary Edition, Winter. I calculated a f/r tire diameter ratio for each of these stock sizes. I used the max and min from those three data points to determine an acceptable range for the f/r tire diameter ratio. It's assumed that the stock sizes would not throw PSM or ABS errors. From there, I looked at the sizes available for the Pilot Sport 4S that fit an 8" front wheel. I calculated a f/r tire diameter ratio (against the 285/35 r18) for all tires that fit an 8" wheel. I found six five sizes that had a f/r tire diameter ratio within the range defined by the stock sizes. These six five tires represented my potential front tire selections. I then considered the overall diameter of the tire. I didn't want a tire that was taller than any of the stock sizes for fear of rubbing. That took the six five choices down to three two. Finally, I considered width. Width gives some indication of contact patch. Since the 285/35 r18 will be the rear tire regardless of the front tire size chosen, the steering dynamic could be slightly modified with front contact patch: more contact patch = less understeer, less contact patch = more understeer. I find my car to have more understeer than I'd prefer. The rear contact patch of the new tire would be slightly smaller (.4") than what I currently have, so that will potentially dial back some of the understeer. To avoid negating the change created by the rear tire or to potentially decrease understeer further, I wanted a front width that was at least equal to or greater than my current stock front tire . That narrowed it down to two one front tire choice. Results: Based on above, the two front tire size that should fit without rubbing, not throw any errors, and potentially provide a driving dynamic with less understeer are 235/40 r18 and 245/35 r18. The 235/40 is the stock 40th Anniversary front tire. Yes, all this work and I came back to "stock size is the recommended size". At least now, I feel it was less than a guess. Interestingly, the 245/35 would be a fatter tire, having .3" more width than stock. Could it rub on full steering lock? Maybe, but I'd be surprised if the tolerances are that tight. Will the extra .3" of width in front coupled with the decrease of .4" of width in back turn this car into a 911 of old where I'm going to oversteer my way *** backwards into a field somewhere? Maybe, but that's more of a reflection of the car's real weak point: the nut holding the steering wheel. Hope this helps someone. Take it with as many grains of salt as necessary. I hopefully haven't made any ridiculous assumptions or stupid math mistakes somewhere that negates all of this work. Data and details are in the attached file, if you want to see how I came to my conclusion. Be aware that each snapshot from TireSize.com has front/rear tire combinations, not comparisons of one size of front tire to another. Now I'm off to decide whether I want to experiment with the fatter front tire at the risk of having to buy front tires twice. -Brad Tire size comparison.pdf
  3. Resurrecting an old thread... I'm moving on from a set of Pirelli P-zeroes that I "inherited" when I bought my 2004 Anniversary Edition (235/40r18 and 295/30r18). I want to get a set of Pilot Sport 4S, but they still don't make the proper rear size. Smokeman909, what did you end up doing? My winter tires are 235/40r18 and 265/40r18. They don't throw any errors (ABS, PSM, or TPMS). They give up 1" in height, but 1.2" in contact patch width. For winter, that's fine. It's arguably beneficial. But for summer, I'd prefer not to give up the width. I'm considering keeping the front at the recommended 235/40r18 and stepping down to a 285/35r18 for the back on the new summer tires. According to online calculators, that tire gives up .4" in contact patch width, but 1.4" in height. My understanding is that changing the height of the tires, or more specifically changing the proportional height of the front to rear tires, will cause errors to be thrown by ABS, PSM, or TPMS. The 285/35r18 is a 5.1% size change compared to the 295/40r18. Has anyone successfully run this size of tire? Thanks in advance for any input. I'm hoping to take advantage of Discount Tires' 15% discount this weekend and set myself up for summertime!
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