northstretch Posted August 18, 2009 Share Posted August 18, 2009 Hey chaps, Again, done a search but cant find the answer. The official pressue (as far as im aware) for the stock 17's is 36psi all round. Please correct me if im wrong. I've just removed the 255's from my rears and they were worn down to the camber in the middle where as the outside edges seems to have some life left in them. This suggests over inflation even though i've always kept them bang on at 36psi. Possibly due to my car being an NA? Less weight? Anyway i replaced them today with some 275/40/17 Kumho KU31's. Anyone got any suggestions what PSI i should be running? The garage filled them to 32psi, i thought i'd leave them at that and see how i get on unless anyone thinks thats too low? Im guessing a wider tyre needs less pressure as it has more surface area? Cheers Lee Quote Link to comment Share on other sites More sharing options...
nigelboyne Posted August 18, 2009 Share Posted August 18, 2009 thats fine i run 32psi for town and generall driving 275 look well on the standard wheel. high speed runs 40psi Quote Link to comment Share on other sites More sharing options...
martini Posted August 18, 2009 Share Posted August 18, 2009 I think mine are 34 PSI all round (18" though). Quote Link to comment Share on other sites More sharing options...
Steviekid Posted August 18, 2009 Share Posted August 18, 2009 Im guessing a wider tyre needs less pressure as it has more surface area? At the same tyre pressure, the contact patch area is exactly the same no matter what width of tyre you put on, it's just a different shape. Quote Link to comment Share on other sites More sharing options...
Ian C Posted August 18, 2009 Share Posted August 18, 2009 At the same tyre pressure, the contact patch area is exactly the same no matter what width of tyre you put on, it's just a different shape. Are you sure?! So the spacesaver at 32psi has the same amount of rubber on the road as a set of 335's? Quote Link to comment Share on other sites More sharing options...
Stephen G Posted August 18, 2009 Share Posted August 18, 2009 i guy at a tire depo told me psi of your wheel should be double your rim size Quote Link to comment Share on other sites More sharing options...
Wez Posted August 18, 2009 Share Posted August 18, 2009 Have you increased the tire width at the front as well? I use to run Khumo Supras 275 on stock 17" rears and the handling was shockingly crap, went back to stock sizes and Eagle F1 GSD3 for road use. Quote Link to comment Share on other sites More sharing options...
nigelboyne Posted August 18, 2009 Share Posted August 18, 2009 so 20mm made the car handle bad. 275 look better on the car and handle a lot better i am searching for 315/35 17 to go on the back. must be something wrong with your car suspension set up cambor toe etc Quote Link to comment Share on other sites More sharing options...
Wez Posted August 19, 2009 Share Posted August 19, 2009 so 20mm made the car handle bad. 275 look better on the car and handle a lot better i am searching for 315/35 17 to go on the back. must be something wrong with your car suspension set up cambor toe etc Quote Link to comment Share on other sites More sharing options...
Thorin Posted August 19, 2009 Share Posted August 19, 2009 so 20mm made the car handle bad. 275 look better on the car and handle a lot better i am searching for 315/35 17 to go on the back. must be something wrong with your car suspension set up cambor toe etc Oh dear. What size front's are you running then? Quote Link to comment Share on other sites More sharing options...
johnny g Posted August 19, 2009 Share Posted August 19, 2009 so 20mm made the car handle bad. 275 look better on the car and handle a lot better i am searching for 315/35 17 to go on the back. must be something wrong with your car suspension set up cambor toe etc I take it you don't know much about Wez's car then ... 315/35/17? You must be mental. That's FAR too much rubber on the road. Quote Link to comment Share on other sites More sharing options...
northstretch Posted August 19, 2009 Author Share Posted August 19, 2009 Have you increased the tire width at the front as well? I use to run Khumo Supras 275 on stock 17" rears and the handling was shockingly crap, went back to stock sizes and Eagle F1 GSD3 for road use. I've currently got 235/45/17 pilot sports on the front but once they wear out i'll up them to 245/45/17 Kumho's so i have the same brand all round. I drove to work with the new rears on today. There were a little slippery last night when i went to the Reading Jap Meet but today they feel awesome. Quote Link to comment Share on other sites More sharing options...
Scott Posted August 19, 2009 Share Posted August 19, 2009 Are you sure?! So the spacesaver at 32psi has the same amount of rubber on the road as a set of 335's? As i understand it thats correct. It sounds insane but someone drew up footprints of different width tyres on the MR2 forum. The wider the tyre the wider the footprint but the shorter it became. The area of the footprint was the same regardless of the tyre size. Quote Link to comment Share on other sites More sharing options...
johnny g Posted August 19, 2009 Share Posted August 19, 2009 As i understand it thats correct. It sounds insane but someone drew up footprints of different width tyres on the MR2 forum. The wider the tyre the wider the footprint but the shorter it became. The area of the footprint was the same regardless of the tyre size. Why do rally cars use skinny tyres in the snow then? Quote Link to comment Share on other sites More sharing options...
Scott Posted August 19, 2009 Share Posted August 19, 2009 Why do rally cars use skinny tyres in the snow then? Thats all to do with pressure, different kettle of fish Quote Link to comment Share on other sites More sharing options...
Hermit Posted August 19, 2009 Share Posted August 19, 2009 Different kettle? Or different fish? It matters! Why do rally cars use skinny tyres in the snow then? Partly the same reasoning - with a longer, narrower footprint the front part can clear some snow giving the back part a better surface to work on. But a snow tyre's profile is also different, making it push the snow out to the side better than a more square profiled tyre would. But see this about contact patch area being the same... http://www.performancesimulations.com/fact-or-fiction-tires-1.htm It's a black art Quote Link to comment Share on other sites More sharing options...
Chiefgroover Posted August 19, 2009 Share Posted August 19, 2009 Tyre pressure is revelant to load rating. If your load rating goes up from stock, you can use less pressure, and if it goes down - your mad it will roll the tyre too much and no extra amount of air will make it handle good. 1/2 to 1 psi per load index rating is what I use depending on the vehicle. heavy cars 1/2 psi and lighter stuff 1psi. Works for me. Quote Link to comment Share on other sites More sharing options...
mikeyb10supra Posted August 19, 2009 Share Posted August 19, 2009 36psi all round Quote Link to comment Share on other sites More sharing options...
nigelboyne Posted August 19, 2009 Share Posted August 19, 2009 245/45/17 front Quote Link to comment Share on other sites More sharing options...
Steviekid Posted August 19, 2009 Share Posted August 19, 2009 (edited) Are you sure?! So the spacesaver at 32psi has the same amount of rubber on the road as a set of 335's? Yep. As long as the tyre pressure and the weight of the car stay the same. Edit: at least that's what my mate with a engineering degree used to bang on about! The article above (which I can't be bothered reading all the way through seems to say different). Edited August 19, 2009 by Steviekid (see edit history) Quote Link to comment Share on other sites More sharing options...
Hermit Posted August 19, 2009 Share Posted August 19, 2009 Lol, I haven't read it all either, but it looks like good info Quote Link to comment Share on other sites More sharing options...
Ian C Posted August 19, 2009 Share Posted August 19, 2009 Yep. As long as the tyre pressure and the weight of the car stay the same. Edit: at least that's what my mate with a engineering degree used to bang on about! The article above (which I can't be bothered reading all the way through seems to say different). Well then lets all fit 165's because they are dirt cheap and apparently have the same size contact patch There are many types of engineering, and you can still be wrong even with a degree Quote Link to comment Share on other sites More sharing options...
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