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Determining proper tire pressure
#1
Guys, it is amazing what so-called professional tire stores tell customers about proper tire pressure.  I have a 80k 2010 car and the service manager immediately went to the door panel to see what the manufacturer recommended for tire pressure.  WRONG!  That's the pressure recommended for the tires provided with the car when sold new.  Every tire model has its own unique profile/design and the tire pressure changes whenever that tire model goes onto various different vehicles.  Tires recommend maximum tire pressure but that is NOT what the pressure should be.  So...how do you determine optimum tire pressure?
Start with the obvious: you want the tire to have maximum contact with the road and you do that by examining the tread contact with the pavement.  Here's how I do it: a run a line of water in front of the tire and drive the car over the water far enough that the water left by the tire track is completely dry.  Examine the water mark across the tread: when tire pressure is optimum, the water pattern when the track just reaches dry-ness should be equal across the entire tread.  If the water drys first in the center and last at the edges, then the tire is underinflated.  If the water pattern drys last in the center and first at the edge, then the tire is overinflated.  Adjust tire pressure so that you have a consistent, even pattern.  Do this for all four tires and the pressures should be fairly equal.  You can average the four if they are not too far off.  Some cars may have such a weight different from front to back that you may have to vary tire pressure front and back. 
I've been doing this for 20+ years after being told this by a former tire mechanic.  Most tire stores have never heard of this and refuse to believe it.  It works!

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#2
Good technique. I go to a parking lot, put a line across the tread of my tires with chalk, then drive forward. After driving a few feet, I get out and check the chalk line, same method as you checking with water, if outer edges still have chalk on them, its over inflated, if the center still has chalk, then its under inflated. I installed a small air compressor behind my front bumper on my Raptor to help keep my tires inflated, really helps with determining correct tire pressure because I can inflate or deflate if necessary.

Tom
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#3
Never knew of these techniques so I'll have to try them.

If the same size tire and type i.e. radial, is put on the car, irregardless of the manufacturer, wouldnt the tire pressures be the same, or close to same?
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#4
Maybe, if the air pressure were based on contact area. However, you don't know what kinds of games car manufacturers are playing to meet EPA fuel mileage requirements. The recommended tire pressure is likely to be more based on fuel mileage, than contact area or tire wear, as long as the pressure is within safe operating limits.

You can't hide the tire pressure recommendation like they were trying to hide emissions testing algorithms in the ECU programs Smile

I like both methods, very innovative, and more immediate than waiting until you have enough miles on the tires to measure tire wear with a tread depth gauge. I would recommend still measuring tread depths, as they can also indicate other problems, such as alignments. You have to take driving styles into account, though. Driving corners fast will result in more outside tread wear, or if you take corners to the left faster than corners to the right (as most people driving left-hand drive cars do).



“If you can't explain it simply, you don't understand it well enough.”
--Albert Einstein
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#5
(02-07-2020, 01:29 PM)Don C Wrote: Maybe, if the air pressure were based on contact area. However, you don't know what kinds of games car manufacturers are playing to meet EPA fuel mileage requirements. The recommended tire pressure is likely to be more based on fuel mileage, than contact area or tire wear, as long as the pressure is within safe operating limits.

You can't hide the tire pressure recommendation like they were trying to hide emissions testing algorithms in the ECU programs Smile

I like both methods, very innovative, and more immediate than waiting until you have enough miles on the tires to measure tire wear with a tread depth gauge. I would recommend still measuring tread depths, as they can also indicate other problems, such as alignments. You have to take driving styles into account, though. Driving corners fast will result in more outside tread wear, or if you take corners to the left faster than corners to the right (as most people driving left-hand drive cars do).
You know, I never thought about the manufacturers adjusting tire pressure for better gas mileage, but that sure makes sense!  An over-pressurized set of tires usually has lower rolling resistance (better mileage) but poorer traction on the road.

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http://midlifeharness.com

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#6
The above info is what makes this site great. Learn something new everyday. Thanks midlife and vi.

Kilgon


"The only dumb question is the one not asked"
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#7
Excellent information here. Especially good for our cars because many of us are not using anything close to stock tires.

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