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Old 07-28-2017, 12:07 PM   #1 (permalink)
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What tire pressure should I run?

I have a 2005 Civic with Kumho Ecowing KH30 tires and I need help deciding what air pressure to run in the tires. Honda recommends 30 PSI. I am currently running 38 PSI cold in all 4 tires, and I am wondering if that is a good pressure. My priorities are safety/handling, long tire life and gas mileage. 38 PSI seems to work very well, but I am concerned about wearing out the center of the tires from too much pressure. What pressure should I run? Thanks!

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Old 07-28-2017, 12:38 PM   #2 (permalink)
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You won't have any issues with wearing out the center. I typically run 50 psi in all my tires and I don't have problems with wearing the center.
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Old 07-28-2017, 01:15 PM   #3 (permalink)
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Found this Report by the University of Kansas:

A Service of The University of Kansas Transportation Center for Rural Transit Providers

Disadvantages of over-inflation.

An over-inflated tire is stiff and unyielding and the size of its footprint in contact with the road is reduced. If a vehicle’s tires are overinflated by 6 psi, they could be damaged more easily when running over potholes or debris in the road. Over-inflated tires cannot insulate road irregularities well, causing them to ride harsher. However, higher inflation pressures usually provide an improvement in steering response and cornering stability up to a point. This is why participants who use street tires in autocrosses, track events and road races use higher than normal inflation pressures. But your transit vehicle will handle just fine at recommended tire pressures, and your tires will last longer. Again, the pressure must be checked with a quality air gauge as the inflation pressure cannot be accurately estimated through visual inspection.
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Old 07-28-2017, 04:22 PM   #4 (permalink)
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Quote:
Originally Posted by Daox View Post
You won't have any issues with wearing out the center. I typically run 50 psi in all my tires and I don't have problems with wearing the center.
Thanks, good to know. So what is the disadvantage to running a higher pressure then besides a less smooth ride? What effect does the higher pressure have on handling, stopping distance and tire wear? Thanks, I am okay with a less smooth ride, but I am NOT okay with tire wear or diminished handling/stopping performance.
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Old 07-28-2017, 05:01 PM   #5 (permalink)
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From what I have read, increasing the tyre pressures reduce the grip and the braking ability. By increasing the inflation pressure you reduce the tyre's footprint which is your only contact with the road. Anyone who has had a puncture and tried to brake will know how the deflated tyre grips and pulls you to that side. Okay, higher pressures increases the steering response but again that is because it has reduced the grip. It is the reduced grip that gives you the increased fuel figure.
Here in the UK there is a lot written about over-inflated tyres increasing wear in the centre of the tread.


This next image shows the footprint with reference to the pressure.



I checked my tyres today and the maximum safe pressure, as marked on the sidewall is 44 psi. I run mine a couple of pound over the manufacturers recommendation but that is all (34 instead of 32).
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Old 07-28-2017, 05:33 PM   #6 (permalink)
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Quote:
Originally Posted by JockoT View Post
Found this Report by the University of Kansas:
Over-inflated tires cannot insulate road irregularities well, causing them to ride harsher.
I'll concur with this.

I finally dropped the pressure down on my Miata's tires this morning. I was running 35psi, while the recommended pressure is 26psi...and I have larger/wider tires. At 35psi I felt way too much of the road. Even rolling over the paint lines on the road, it was jarring and could be felt throughout the car. It's much smoother at 28psi.

I'm interested to see if I lose my 1 or 2 MPG's that I have over the normal for it. It will suck if it does, but I don't think it's worth the harsh ride quality.

OTOH, I keep my Metro at 40 or so, and don't have the same issue. Of course, they're a tall, narrow tire vs a semi-low profile, wide tire.
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Old 07-28-2017, 06:12 PM   #7 (permalink)
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Take the weight on a single tire divide it by the psi = the size of the contact patch . Eg my dodge 2500lbs rear axle ÷2 =1250÷side wall max+20%96psi =13.02^" Vs proper air for flat even wear@60psi =20.8^" which one will work better under anything other than straight line?
The performance gained in cornering from raised psi is the over inflated tire being convex compensating for the lack of camber and less slip angle (the difference between how much the wheel turns Vs how much the tred turns)
Eg. the tred is pushed sideways in a hard corner,
Over inflated stock camber, when it will run on the center and sholder.
Data plate/stock camber , sholder and side wall and some center.
Low air will fold under running on the sholder and sidewall when low enough the rim makes contact normally resulting in a flip.
Proper air with proper camber, will run flat.shoulder to shoulder

If you want the ultimate advice get a $20 Infered temp gun. The air is proper when the tire reads the same +-2°f across the tread. As in the formula above this psi # is in a constant state of flux.as you change what or how many things in the car and burn gas so will the weight distribution and volume . Go for 1/2 tank and normal load to get your psi/temperature

One more thing to consider is excessive shock loading/jarring causing abnormal suspension part wear= higher maintenance cost and slower cornering speed (slopy steering add RR )
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Old 07-28-2017, 08:41 PM   #8 (permalink)
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Quote:
Originally Posted by gumby79 View Post
Take the weight on a single tire divide it by the psi = the size of the contact patch . Eg my dodge 2500lbs rear axle ÷2 =1250÷side wall max+20%96psi =13.02^" Vs proper air for flat even wear@60psi =20.8^" which one will work better under anything other than straight line?
The performance gained in cornering from raised psi is the over inflated tire being convex compensating for the lack of camber and less slip angle (the difference between how much the wheel turns Vs how much the tred turns)
Eg. the tred is pushed sideways in a hard corner,
Over inflated stock camber, when it will run on the center and sholder.
Data plate/stock camber , sholder and side wall and some center.
Low air will fold under running on the sholder and sidewall when low enough the rim makes contact normally resulting in a flip.
Proper air with proper camber, will run flat.shoulder to shoulder

If you want the ultimate advice get a $20 Infered temp gun. The air is proper when the tire reads the same +-2°f across the tread. As in the formula above this psi # is in a constant state of flux.as you change what or how many things in the car and burn gas so will the weight distribution and volume . Go for 1/2 tank and normal load to get your psi/temperature

One more thing to consider is excessive shock loading/jarring causing abnormal suspension part wear= higher maintenance cost and slower cornering speed (slopy steering add RR )
Great point about wear on the suspension parts from the bumps/vibration, I wondered about that myself. I have been running 38 PSI in the tires since I got the car with 15K miles on it. It now has 227,700 miles on it, and the front end is starting to show its age. I just changed the front control arm bushings and all 4 sway bar links, as they were starting to wear out. The lower ball joints are also starting to develop play, I am going to change those this weekend, and the CV axles are starting to click. Besides that though, all the suspension is original. I don't know that those parts failing even has anything to do with the high tire pressure though. The car lived almost its whole life in Chicago, and their roads are horrible and full of large pot holes, so that probably has a lot to do with it. But maybe if I ran the recommended 30 PSI in the tires the suspension would still be original and in good shape. Who knows, either way I can't complain about the longevity of the suspension for just starting to give me issues with this many miles on it though.
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Old 07-29-2017, 12:11 AM   #9 (permalink)
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Quote:
Originally Posted by gumby79 View Post
Take the weight on a single tire divide it by the psi = the size of the contact patch.
Not true. One can research it or measure it themselves. I've done both.
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Old 07-29-2017, 02:46 AM   #10 (permalink)
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A quick answer would be 45 PSI. That's what I put in the tires on my mother's Prius.

Longer answer:
Go ahead and experiment! I have 55-60 PSI in my junkyard tires. They last me years and wear evenly. In my experience, some tires will wear out the center at higher pressures. Blizzaks are one example, they don't like high pressures. Others will do just fine. I don't know how much, if at all, I've reduced the tire's traction. I'm pretty sure I have more grip than the smaller original size tires at correct pressures. I'm still on the original shocks, springs and dry-rotted bushings at 239k miles. Ride quality and noise isn't great, but it's a gutted '91 Civic Wagon.

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