The "correct" tire pressure --- NOT A SCIENCE!!!!

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285
Location
nyc
I'm pretty dumb---it took me a couple of years to come to the realization that there is no such thing as a "correct" tire pressure. variables included, personal taste--do u like squishier ride or firmer? What size are the tires? what size are the wheels? what kind of tires?? what is the tire AGE (i find, remarkably, that older tires require MORE tire pressure) FOR ME, the driver's side FRONT gets 39 PSI passenger FRONT gets 37 both rears get 35. the tire wear appears pretty even on the front but there is a center-wear bias on the rear tires so i will need to lower the pressure on the rear. also, my tires are tripletreds on a 15X7 alloy wheel , the size is 215/60/15 which was upgraded from the OEM of p185/65/15 on a 2007 Corolla. the manufacturere recommends 30psi for front and rear.
 
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19,528
Location
Lake Forest, CA
For daily drives, I like higher pressure on front tires for front wheel drive cars, for rear wheel drive cars I like higher pressure on rear tires. I found that the recommended pressures are usually low for ride comfort, increase pressure a few PSI makes the car handle better and it also improve MPG. For long trips, more than few hundreds miles, I increase pressure in both front and rear tires a few PSI more, because I sometimes drove at speed over 100 MPH if conditions permit.
 
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4,450
Location
Kuwait
I've got 265/70-16 tires on my truck and the manufacturer recommends 26 PSI. With 26, I find the outer side of the tires wear out much quicker (along with the rest of it of course), the truck is rather sluggish and fuel economy is terrible. After reading an article on tire pressures, I set all my tires to the maximum cold pressure (44PSI). Significantly better acceleration and handling, smoother ride and a gain of 5.5 MPG with a maintenance dose of MMO. The tires also run cooler.
 
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3,845
Location
Somewhere in the US
Some thoughts:
 Originally Posted By: mareakin
........ on a 15X7 alloy wheel , the size is 215/60/15 which was upgraded from the OEM of P185/65/15 on a 2007 Corolla. The manufacturere recommends 30psi for front and rear.
According to Tire Guides, in 2007 only the CE model of the Toyota Corolla came with P185/65R15's, but they came with 15X6" wheels. I'm assuming they are aftermarket wheels if they are 15X7"'s. But if not, be aware that the 6" rim width is the smallest rim that a P215/60R15 fits on and may be part of the source of the center wear. Another thought: THEE most important thing about tires is load carrying capacity - and inflation pressure is a major factor when it comes to that! When vehicle engineers size tires and select inflation pressures, it's where they start. One of the lessons learned from the Ford / Firestone situation a few years ago is that the load carrying capacity of the tire's size / inflation pressure combination has to be larger than previously thought. In other words, there has to be some unused capacity. The load carrying capacity of a P185/65R15 at 30 psi is 1088#. That means the load carrying capacity of the front tires was 2176. At the published GVW of 3585# and assuming a front / rear weight split of 60 / 40, the front load on the tires is 1971#, so the tires have about a 10% unused capacity - which seems a little small for my taste. To carry the same load a P185/65R15 does at 30 psi, a P215/60R15 needs to have only 20 psi. (Actually the chart doesn't go that low, so I'm interpolating - plus I do not recommend anyone ever use less than 26 psi in passenger car tires, because that the pressure it is possible to unseat the bead when doing emergency manuvers.) So using 39 to 35 psi is ...... well ..... way more load carrying capacity than the original tire size / pressure combination. As indicated, this may also contribute to the center wear. BTW, the tire load tables are predicated on the deflection being the same for all points on the table. One of the fallouts of that is that the footprint size will be the same for all those points. This means that using proportionally more inflation pressure reduces the size of the contact patch. Plus, more inflation pressure raises the spring rate of the tire. Changing the pressure split would change the handling characteristics. So using a 38 / 35 spilt on a car calling for 30 / 30 is going to result in a car with less understeer. Plus using a higher pressure is going to increase what we tire engineers call "aligning torque". It's the force that pulls the tire back into position when the steering wheel is turned. But the inverse of that is that less slip angle develops for a given steering input - and that results in a more precise steering feel to the car - call it "crispness". So overall, the car is going to feel more balanced and precise. However, this has a down side. Understeering cars communicate their reaching the limit of adhesion by giving you plenty of warning. Increase the inflation pressure, and the limit of adhesion is approached more abruptly. Make the car have less understeering and the approach to the limit of adhesion is less noticeable. Overall, vehicle engineers consider this less safe, as the warnings that the limit of adhesion is being approached become less obvious. So that's some of the science behind the tire pressure thing. A word of caution: Vehicle engineers spend coutless hours driving their cars on their test tracks in order to find the little handling quirks. So be cautious about using an inflation pressure different than what is specified.
 
Messages
950
Location
Loveland, Ohio
Except in the case of Ford/Firestone in the Explorer fiasco. I look at them as guidelines and adjust them for your own preferences. Increasing the pressures a bit improves handling, mpg, and tire life usually. Mfrs tend to pick lower pressures for softer rides, not ideal in my opinion.
 
So basically what you are saying Capri is that if one messes around with their tire pressure they might gain some handling at the expense of the inherent safety of understeer. I used to mess around with tire pressure mostly to increase mpg. The guys at the garage followed the placard and let some air outta my tires to my dismay. Decided to leave it and see if anything changed. Guess what? Car rides noticeably better and fuel economy stayed about the same.(It actually went up but I attribute that to higher average temperatures.)
 
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Messages
14,505
Location
Top of Virginia
This is interesting. I've played with inflation pressures on my own '07 Corolla (using the stock 195/65R15 size), and actually find that I like the OE 30/30 best (which is unusual for me). Increasing inflation pressure quickly added ride harshness, and I actually get best fuel economy at the OE pressure spec. On my minivan, it specs 36/36, and I run them at about 38/36.
 
Messages
10,008
Location
Upstate NY
My Buick needs all the help it can get in the handling department, so my tires are run at sidewall max for the front, and 2-3 psi less for the rear. No change in ride quality, but it handles a lot better. The tires are also less prone to rolling over onto the sidewall in turns, improving tire life, too.
 
Messages
1,784
Location
Bonnyville, AB
I just inflate them until the radial sag is gone, usually 3 pounds over all around. In winter, I wait until the first -40 day I come across and bring them up to placard pressure so through the temperature range I'm not running tires low.
 
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35,998
Location
ME
I read a theory that if you overinflate your tires the shocks/struts take more abuse, which is converted to heat. Tire deflection gets pushed back into the pavement which helps MPG! I also inflate my tires in winter on the coldest crispest day I can find... because of the low absolute humidity.
 
Messages
14,505
Location
Top of Virginia
I've read the opposite -- that if the tires are inflated too much, small road imperfections can no longer be taken up by the tires and must be taken up by the suspension dampers, reducing economy. Any truth to either of these opposing claims?
 

mva

Messages
765
Location
BC, Canada
Higher pressure = lower rolling resistance and better mpg Try riding a bike at 30 psi verses 80 psi to prove it to yourself. If you think the suspension counters this effect then try it on a bike with suspension. But the effect on mpg on a car is small enough that it will be difficult to detect.
 
Messages
12,385
Location
Northern CA
 Originally Posted By: Jason Adcock
I've read the opposite -- that if the tires are inflated too much, small road imperfections can no longer be taken up by the tires and must be taken up by the suspension dampers, reducing economy. Any truth to either of these opposing claims?
I doubt that you could put that much pressure in a tire without it coming apart. Here's some interesting test work. Simple but well done and the guy is very careful to not claim to know more than he does. http://metrompg.com/posts/tire-pressure-rolling-resistance.htm It looks like on the two cars he tested there was no advantage to going over 50 psi and little advantage to going over 40 psi. It would have been interesting so see some runs at 70 and 80psi, but I wouldn't have knelt beside a passenger car tire while putting 80 psi in it so I'm not complaining. Obviously steel wheels rolling on pavement don't roll as easily as pneumatic ties, so there is some level of tire hardness and stiffness that's too high for max economy. Maybe Capriracer can shed some light on this.
 
Messages
10,008
Location
Upstate NY
^ There's truth there. Extreme example: Riding my road bike with the tires at 80 psi means noticeably more effort than riding with my tires at the 120 psi sidewall max. The bike coasts farther and reaches faster speeds coasting downhill at 120 psi also. But I feel the road imperfections much more at 120 than at 80. In the car I see the effect that 15-20 psi has. The placard for my car is 32/30, while the tire sidewall is 44. I have driven on the same set of tires at 30 psi, and at 44 psi. They definitely roll more easily at 44 psi. And the road comes through more.
 
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4,844
Location
Saskatchewan
 Originally Posted By: crinkles
follow the door placard... we do not know more than vehicle manufacturers or auto engineers...
Then why did they put those horrible RS-As on my car before they gave it to me? I've been running 35/32 on my car. 32/32 is the recommended pressure, and I've played with everything from 28/28, where wheel hop disappears, to 35/32, where it corners well.
 
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39,792
Location
Great Lakes
 Originally Posted By: rpn453
 Originally Posted By: crinkles
follow the door placard... we do not know more than vehicle manufacturers or auto engineers...
Then why did they put those horrible RS-As on my car before they gave it to me?
Because at the end, the engineers got overridden by the finance department.
 
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