03slatt Posted March 6, 2020 Report Share Posted March 6, 2020 i'm sure this has been asked before but i've done a search and I cant find it. Costco have a good deal on at them moment for tyres. my tyres are the 235 50 17 size but they only do PS2's for £150 odd each if i change to 235 45 17 I can get PS4's for £92 each my question is will 235 45 be ok ? I"m sure it will be but just wanted to get peoples opinions. Link to comment Share on other sites More sharing options...
minimoog Posted March 6, 2020 Report Share Posted March 6, 2020 The 45s will be 3.8% smaller in diameter, which is a smidge more than recommended on assorted tyre size comparison sites, which tend to recommend no more than a 2-3% difference. Probably not the end of the world though. Link to comment Share on other sites More sharing options...
Del_tl1000r Posted March 8, 2020 Report Share Posted March 8, 2020 As minimoog stated the overall diameter will be smaller so will be doing more revs. This may upset your speed and fuel consumption but I'm not sure. The overall diameter will be 23mm smaller so the car will sit lower by approx 11.5mm and the gap between the tyre and the edge of the wheel arch will increase by approx. 11.5mm. It could look a bit odd. Hop this helps. Link to comment Share on other sites More sharing options...
mid-life Posted March 8, 2020 Report Share Posted March 8, 2020 always worth taking into account that new tyres vs old @ say 2mm will be 12mm approx. greater diameter. Link to comment Share on other sites More sharing options...
Del_tl1000r Posted March 8, 2020 Report Share Posted March 8, 2020 25 minutes ago, mid-life said: always worth taking into account that new tyres vs old @ say 2mm will be 12mm approx. greater diameter. Don't really understand what that means. The dimensions I stated earlier are based on new tyres, the same width but with different profiles. Link to comment Share on other sites More sharing options...
mid-life Posted March 8, 2020 Report Share Posted March 8, 2020 15 minutes ago, Del_tl1000r said: Don't really understand what that means. The dimensions I stated earlier are based on new tyres, the same width but with different profiles. Simply means that there can be as much as 12 - 14 mm difference in the diameter between worn and new tyres. So assuming that you are replacing old for new tyres, then some of the diameter difference (if you use a different profile) is mitigated by the increased diameter due to the deeper tread. Link to comment Share on other sites More sharing options...
Del_tl1000r Posted March 8, 2020 Report Share Posted March 8, 2020 Yes I get that. The OP is asking the differences in the tyres. The sizes I stated are correct for new tyres. Link to comment Share on other sites More sharing options...
mid-life Posted March 8, 2020 Report Share Posted March 8, 2020 22 minutes ago, Del_tl1000r said: Yes I get that. The OP is asking the differences in the tyres. The sizes I stated are correct for new tyres. LOL .. yes I understand what you were saying too. I'm just making the point that tread wear can be quite significant to the overall diameter and therefore the "look" of the wheel on the car and accuracy of speedo's etc. Link to comment Share on other sites More sharing options...
Del_tl1000r Posted March 8, 2020 Report Share Posted March 8, 2020 Well I'm glad that's sorted out. I was just about to get my handbag out! 👜 Link to comment Share on other sites More sharing options...
zcacogp Posted March 8, 2020 Report Share Posted March 8, 2020 Some people complain that the 987 was quite over-geared so smaller tyres may help reduce this a little. One thing I noticed when I went from 18" wheels to 17" wheels was that the underside of the nose was MUCH less prone to gently catch on the local speed bumps. Apparently the rolling radius of the 18" wheels with standard tyres is lower than that of the 17" wheels with standard tyres which was enough to make a difference. I'd therefore not want to choose any tyres for the 17" wheel which reduced the rolling radius, but it also suggests that the rolling radius of the 18" wheel is smaller than that of the standard tyre on the 17" wheel anyway so going down a size in tyre on the 17" wheel may not be a problem. Hope that explanation made sense! Link to comment Share on other sites More sharing options...
03slatt Posted March 8, 2020 Author Report Share Posted March 8, 2020 Ok thanks for the help everyone, I'm probably going to end up buying all 4 tyres now so as my fronts are 10 years old although they look new. I think it comes down to the price as itll be almost £200 cheaper for four better tyres(ps4>ps2) I will have to ask costco if they will actually fit them though as I've heard they dont fit tyres that aren't the correct size. Then I'll just have to go to mr tyre. Cheers Tom Link to comment Share on other sites More sharing options...
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