Different PSI for different tire/wheel combo?

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If you change to a different size wheel and/or tire size, should you alter the PSI you run in the tire? For example, would a 215/45R17 tire on a 7" wheel (stock) require the same PSI as a 205/55R16 on a 6.5" wheel (aftermarket)? I'm just using these sizes as examples, so they may not be real, but it conveys my question. Should I still use the vehicle manufacturer's recommended PSI, or should the PSI be higher or lower?
 
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Best way (in racing terms) would be to use a probe-type tire pyrometer (the kind you stick into the rubber, not a surface temp reader) and measure temps across the tread at different pressures, but it also depends on what you are looking for in terms of ride, tire life, and load capability etc. I'd at least not go below the stock psi rec'd by the manufacturer, and use it as a base. Then go up from there depending on load, speed driven, and tire life and ride quality wanted.
 

ZiTS

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Oops, I just realized that I labeled the stock and aftermarket sizes backwards. Not that it matters, but I'm anal like that... The actual sizes I've got are 205/50R16 (stock), and 215/40R17 (aftermarket).
 
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When you change tire size you have to compensate for the difference in load capacities, so you need to recalculate the new pressure based on the load carrying capacity of the old tire size at the inflation specified - not a trivial calculation. The problem with the pyrometer approach is that it is only good for the conditions you just experienced. Change conditions - change answers! So don't use that for anything but the race track. Same with the "chalk" method. It only applies to what you just did. Hope this helps.
 
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You'll need to run higher pressures. I suspect the load ratings for the 17" is significantly lower and you'll want to protect the rims with the low profile tire by giving it more resistance to sidewall flex. It's the same diameter tire as I use and I went with a 225/50/16 on a 7.5" wide wheel. I actually get my best results by running stock (32 psi) pressure in front and a bit less in back but in a slightly lowered car. I could use less as the 225/50 has a higher load rating and the wheel width is on the higher side of spec (Stabalizes the sidewall). It's a looks vs peformance thing. Unless you've got a a highly refined suspension like a BMW, you won't benefit from a super low profile tire. It will ride worse, be more prone to wheel damage and can actually handle worse from an inability to track road imperfections as well. Taller wheels usually weigh more also. The key is to get enough wheel width for the tire you choose. In your case, I don't think a 215/45 is going to handle worse than a 225/50 but I don't think it will be better either. The question becomes, apperance vs comfort and wheel protection. You also won't have as large a range of pressures to tweek with the 17 as you should probably be starting at about 35 psi. [ July 18, 2005, 07:48 AM: Message edited by: goodvibes ]
 
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Tire pressure isn't as critical as it once was, as long as you don't underinflate. Modern radial tires do a pretty good job of keeping the tread evenly on the ground compared to old bias ply tires. Unless you have an averloaded pig of an SUV, you tires are probably not running near their load capacity, verify that and you should be able to not be concerned about that. Use your original factory pressure as a baseline and experiment upward abit from there to pick the pressure that gives you the handling and ride combination that you want.
 

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I would think you would want a little higher pressure in the lower profile tires. Otherwise, you might not be very happy after a few railroad tracks and potholes. I have not verified it, but I believe that the higher profile tires that are stock on my saab use a little less pressure in the tires. JMH
 

ZiTS

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Thanks for all the input, everyone. While I was waiting for replies to come in, I called The Tire Rack, as well. This is where I bought the tires, and they (just like BITOG) have always been very helpful. The tech I spoke with stated that because the tires I selected are compatible with the stock size, it should work best to start at the stock PSI and increase 1 or 2 PSI gradually. He added that if I wanted better turn-in, I could up the rear PSI 1 or 2 pounds, understanding ride may get compromised a bit. So, it sounds like everyone is in agreement - I'll start at the stock PSI recommendation, and then increase it incrementally, if needed. You know, I consider myself to be a fairly smart guy with a lot of common sense, yet it hadn't dawned on me before now about the difference relating air pressure to tire/wheel size changes. Makes me wonder (shudder) about all the other millions of cars on the road where the owner has changed to 22" wagon wheels and never even gave the air pressure change a thought. Most people won't even pay attention to the stock tire/wheel air pressure, let alone taking the time to figure out what an aftermarket set would require. [Frown]
 
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quote:
Originally posted by CapriRacer: The problem with the pyrometer approach is that it is only good for the conditions you just experienced. Change conditions - change answers! So don't use that for anything but the race track.
I have to disagree, here. The pyrometer approach can be used in any conditions whatsoever and help you arrive at the correct tire pressures for your car, your driving style, and you driving conditions. What happens is that you may find that you want one set of pressures for driving task A and a slightly different set for driving task B. The old way of just setting cold pressures would leave you with the fuzzy comfort of a single number for the PSI. While the pyrometer can actually educate you about what the car wants with your various tasks and styles. After you experience several situations and measure the temps and adjust the pressures for different circumstances; you will find that all of these gather around a central point. Set the pressure to this central point, and you will be near optimal for the life of those tires.
 
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In these sizes the 16 will have a higher load rating but even higher pressures won't help if you hit the wrong size hole. You wouldn't think that 1/2" of sidewall would be a big deal but when the 205/50/16 is already iffy for the real world, that 1/2" can make a difference. The load rating also doesn't have a thing to do with handling.
 

JHZR2

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quote:
Originally posted by ZiTS: Makes me wonder (shudder) about all the other millions of cars on the road where the owner has changed to 22" wagon wheels and never even gave the air pressure change a thought. Most people won't even pay attention to the stock tire/wheel air pressure, let alone taking the time to figure out what an aftermarket set would require. [Frown]
Unfortunately its likely that the subset of folks that you mnentionare the cause of a disproportionate number of problems these days. JMH
 
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I'd agree with CapriRacer here. The tire pressure would mostly depend on the load rating of the tires. So if both 205/50/16 and say 215/40/17 have the same load rating, then you could keep your tire pressure about the same. If the load rating is lower, then I'd increase the PSI a bit. Of course, you should not be buying tires with a load rating that is lower than the one specified in your owner's manual. Some more anal automakers actually provide the recommended PSI for a number of tire sizes: Tire pressure sticker
 
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