The Caretaker Posted 28August, 2019 Report Share Posted 28August, 2019 Long story short, where talking about a 997 S coupe. I have recently fitted Michelin PS 4's to the rear 305/30/19 and now looking at the fronts, currently 235's. My tyre guy is saying why don't you go for the 245/35/19 as according to him, match up better with the 305's. The trouble is I can't find any flavour of 997 spec that puts a 245 on the front, plenty that run with 305's matched to 235's which include the GT3. Pro's and con's please guys. Quote Link to comment Share on other sites More sharing options...
donR Posted 29August, 2019 Report Share Posted 29August, 2019 I'd also go with the 245s, you will get a little bit more front end grip, a little better braking and a little less understeer, but in the wet it probably wont help. Quote Link to comment Share on other sites More sharing options...
edgy Posted 29August, 2019 Report Share Posted 29August, 2019 One thing I'd cautious of; given the same aspect ratio, the rolling diameter will be affected by this increased width... the ABS system in our cars can be very sensitive to differences in wheel speeds, the worse case is that it triggers ABS ice mode which dramatically impacts your stopping power in a normal situation (read almost non existent) needless to say the consequences can be severe or even deadly. The key is to ensure consistent/relative rolling diameters front to back, I am not sure exactly what the tolerances are mind you. Rennlist search might yield an answer for you? I would ask about to be honest and see if people are doing this successfully or not. Quote Link to comment Share on other sites More sharing options...
Skidmarks Posted 29August, 2019 Report Share Posted 29August, 2019 Porsche specified 235 for a reason ... edgy and Troubleshooter 2 Quote Link to comment Share on other sites More sharing options...
RwGreeny Posted 29August, 2019 Report Share Posted 29August, 2019 58 minutes ago, Skidmarks said: Porsche specified 235 for a reason ... What Skidmarks said! If you want to move your chassis grip around do it through sway bar settings. A 10mm wider front tire won’t instantly give you more bite Quote Link to comment Share on other sites More sharing options...
The Caretaker Posted 29August, 2019 Author Report Share Posted 29August, 2019 2 hours ago, edgy said: One thing I'd cautious of; given the same aspect ratio, the rolling diameter will be affected by this increased width... the ABS system in our cars can be very sensitive to differences in wheel speeds, the worse case is that it triggers ABS ice mode which dramatically impacts your stopping power in a normal situation (read almost non existent) needless to say the consequences can be severe or even deadly. The key is to ensure consistent/relative rolling diameters front to back, I am not sure exactly what the tolerances are mind you. Rennlist search might yield an answer for you? I would ask about to be honest and see if people are doing this successfully or not. It's interesting, Rennlist is suggesting there are guy's that track there cars and prefer the 305/245 setup over 305/235. Regards the rolling diameters I think that is what my tyre guy was iluding to, the CS 2 spec is 295/235 and as I have gone with 305 rears(ticked off by PCM) his theory is it restores the status quo. Can I say upfront, this for me is more about aesthtic's not about performance gains. Having said that I'm also not looking to send the car backwards. Quote Link to comment Share on other sites More sharing options...
edgy Posted 29August, 2019 Report Share Posted 29August, 2019 3 hours ago, Skidmarks said: Porsche specified 235 for a reason ... I agree 1 hour ago, The Caretaker said: It's interesting, Rennlist is suggesting there are guy's that track there cars and prefer the 305/245 setup over 305/235. Regards the rolling diameters I think that is what my tyre guy was iluding to, the CS 2 spec is 295/235 and as I have gone with 305 rears(ticked off by PCM) his theory is it restores the status quo. Can I say upfront, this for me is more about aesthtic's not about performance gains. Having said that I'm also not looking to send the car backwards. I would be researching the rolling diameters for the sizes you're talking about, start with the OEM sizes, then check the variance between those, once you get that established, check the variance in the 245 F/ 305 R combo, if its the same variance, it is probably not an issue. My comment isn't intended to be of the fear mongering kind, however just a consideration you should be aware of... you can trigger the ABS ice mode driving the car on its limits just by your braking technique... even when the tyre sizes are fine. So it is plausible you may have no issues in doing it for 99% of your driving, but you might find out when its too late what ice mode is... ask me how I know The Caretaker 1 Quote Link to comment Share on other sites More sharing options...
The Caretaker Posted 29August, 2019 Author Report Share Posted 29August, 2019 32 minutes ago, edgy said: I agree I would be researching the rolling diameters for the sizes you're talking about, start with the OEM sizes, then check the variance between those, once you get that established, check the variance in the 245 F/ 305 R combo, if its the same variance, it is probably not an issue. My comment isn't intended to be of the fear mongering kind, however just a consideration you should be aware of... you can trigger the ABS ice mode driving the car on its limits just by your braking technique... even when the tyre sizes are fine. So it is plausible you may have no issues in doing it for 99% of your driving, but you might find out when its too late what ice mode is... ask me how I know Thanks very much Edgy ! Unless someone categorically tells me they run 245's and it works, then 235's it will be. edgy 1 Quote Link to comment Share on other sites More sharing options...
Jimmy Posted 29August, 2019 Report Share Posted 29August, 2019 All makes sense Edgy I considered going from 295/30/20 to 305/30/20 on my GT4 as I wanted to go to PS4 but they had to be "N" rated for extended warranty, 295 4S don't come in N rating, next best fit was 305. Karl from Motorsport Dept PCM said Jim Richards GT4 on 305's got an error code at high speeds, so in the end I went with std CUP2. The Caretaker and edgy 2 Quote Link to comment Share on other sites More sharing options...
edgy Posted 30August, 2019 Report Share Posted 30August, 2019 For what its worth, I was looking at a 997 GT2 RS today, that runs a 245/35 19 front tyre, and they put that little fender flare on mind you, but the rear tyre is a 325/30/ 19 NOT a 305 Quote Link to comment Share on other sites More sharing options...
The Caretaker Posted 30August, 2019 Author Report Share Posted 30August, 2019 1 hour ago, edgy said: For what its worth, I was looking at a 997 GT2 RS today, that runs a 245/35 19 front tyre, and they put that little fender flare on mind you, but the rear tyre is a 325/30/ 19 NOT a 305 Okay edgy ! I thought we had just about shut the gate on this one, but now you've flung it open again lol ! Quote Link to comment Share on other sites More sharing options...
1q2w3e4r Posted 30August, 2019 Report Share Posted 30August, 2019 2 hours ago, edgy said: For what its worth, I was looking at a 997 GT2 RS today, that runs a 245/35 19 front tyre, and they put that little fender flare on mind you, but the rear tyre is a 325/30/ 19 NOT a 305 The 2RS runs the same spec as the 7/2 on the rear but a wider front (245 v 235) To confuse matters I had to stick a 245/40/18 on my 6TT at Bathurst this year when the Cup2 235 started to delaminate and we couldn't get a replacement in same size at the track (on the Easter long weekend). It ran fine for the remaining sessions on the weekend matched to a 295/30 rear. Quote Link to comment Share on other sites More sharing options...
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