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Discussion Starter #1
In a fit of EPA giddiness I boosted the front tires to 48PSI cold (at 75F temps in heated garage) considering the outside temps drop to 20F area.. with a drop of 1 pound PSi for each 10 degrees average..
I want 44PSI front in cold weather (sub freezing)cold tire temps for MPG saving.
So my question is:
Are the stock wheels on my 2012 going to be perfectly happy with that 48psi tire on them?
(I am NOT worried about the tires at all rated 51 cold PSI max pressure) [hatchet]
And not a lot of big potholes to cause damage either.. just regular driving..

Just the front are at 48psi. the rear are at 40psi..
(Keeping my 8psi difference for front rear weight on tires...)

[hah] The ride is very reasonable considering the PSI boost.
 

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The trade off between accelerated shock wear, tire wear (the center of the tread will in all likelihood wear quicker), poorer braking and handling doesn't seem worth the minimal mpg increase over the long run. It could result in being an expensive experiment (even if potential tire damage from bad roads or debris is not considered).
 

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Aurelius Pardus
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the wheel will be fine.... not sure what TPMS is set at or if you have it on yours.

Also don't forget that even if it drops in temperature it rises back up when driving long enough, so with that I ask how long your average driving trip is.
 

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Discussion Starter #4
I caved and lowered all around 2psi
So now i am 46psi front and 38psi rear.
With a 5psi gap to max tire pressure i feel better.
Now only 2,500 pounds of pressure are against the rim of wheel
(rim circ approx 4.7 ft times 1/2 width times two sides of rim = approx 2,500pounds of pressure just on rims per wheel where wheel and rim intersect at 46 psi in tire.. [neener] assuming the interface is one half inch wide..
 

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Shiny Metal Titanium
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Jesus. lol. Well I'm sure it'll be fine, just like Geezer said there will definitely be some accelerated tire wear in the center of the tread among other things.

With that said, I actually had a faulty gauge that read 10psi low so at some point I filled my tires up to "40 psi" according to the gauge and drove around with 50 for a while. That made my car handle funny on the highway. I would get blown around from wind or little grooves alot easier. The worst was if I got behind a 18 wheeler the draft from that would start knocking me all over the lane. Never suspected the tires until one day I checked with a different gauge and found them all at 50psi
 

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From experience I can tell that you MIGHT not be able to depend on your tire gauge...I had one reading 6 lbs low...so at times my tires were 6 lbs OVER what I thought they were. So I bought 3 inexpensive dial type tire gauges (diff brands)...already after a few months...one is reading under relative to the other 2.

So how can you know if your tire gauge is reading right? Don't count on the local tire shop.....

Maybe you can find a gauge that sets itself vs the atomic PSI gauge at the Naval Observatory? [facepalm]

I ran a set of Kumhos on another car from what turned out to be 40 t0 46 PSI for their full life (~40K).....44 MAX tires....and they wore almost perfectly across the tread.

As far a front/rear bias...right now I use the same front and back...since the door sticker says to...but it makes sense to run the rears maybe 3-4 lbs lower....less chance the back will break loose. On my car the back is slightly skittish at 42/42 and have had occasion to test it several times...there doesn't seem to be an inherent tendency for the back to slide much.

Right now I'm at 42 front and back....will probably drop to 40 front and maybe 38 rear for serious winter....44 max tires. You can lose much more than the few $ you've saved with mpg gains by making just one mistake?

http://metrompg.com/posts/tire-pressure-rolling-resistance.htm

http://ecomodder.com/forum/showthread.php/mythbusters-tests-tyre-tire-pressure-17151.html

It's also important to check your alignment often...after doing my own on 3 diff vehicles I'd guess that at least 50% of vehicles on the road at any one time are seriously out of alignment. So it's either pay for the lifetime alignment or DIY?
 

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Inflate to max sidewall if you want fuel economy. I have been running 44 psi in my tires and I havent noticed any weird wear patterns but the handling is a little different but not unbearable. I use the fuel economy graphing tool on EcoModder and I can definitely tella difference. I just reinflated today after being down to almost 25 psi on some tires and I removed my rear seat so hopefully I will be back up to the good numbers again soon. I've been averaging 34 mpg lately on the low pressure but I have gotten as high as 42 mpg when everything is in tip top shape.

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Sorry but this is bad advise from the simple fact that as a tire heats up (and pressure increases) it could easily end up exceeding the maximum allowable pressure.
The sidewall pressure is the maximum cold pressure. (1) The engineers allow for the increase above cold when the tires are driven on and (2), the overinflation would result in less flexing and geat buidup than normal. My truck OEM tires were marked 35 PSI and the door sticker pressures are 32 front and 35 rear. I had no failures through the life of the tread, but replaced them with AT tires (rated at 44 max) for the better traction in our every other year major snow storm. If it snows I drive the truck just because I like 4500 pounds around me when somebody decides to pull out in front of me, etc.
 

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The sidewall pressure is the maximum cold pressure. (1) The engineers allow for the increase above cold when the tires are driven on and (2), the overinflation would result in less flexing and geat buidup than normal. My truck OEM tires were marked 35 PSI and the door sticker pressures are 32 front and 35 rear. I had no failures through the life of the tread, but replaced them with AT tires (rated at 44 max) for the better traction in our every other year major snow storm. If it snows I drive the truck just because I like 4500 pounds around me when somebody decides to pull out in front of me, etc.
Took the words right out of my mouth.

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The max side wall pressure takes into account the pressure increase due to temperature so you are perfectly safe running the side wall rating.

As an engineer any pressure containing vessel is designed with a safety factor of at least 2. Not that i am recommending it, it does mean you could inflate them to 88 psi without the tire rupturing.
 

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C2H5OH
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The max side wall pressure takes into account the *constant pressure increase due to temperature so you are perfectly safe running the side wall rating.

As an engineer any pressure containing vessel is designed with a safety factor of at least 2. Not that i am recommending it, it does mean you could inflate them to 88 psi without the tire rupturing.
Your 'factor' does not take into account that the pressure vessel is of varying volume. It only has a constant volume at rest or with no loading.
Which means that road conditions can certainly have an adverse effect of the tires, most all times in a bad way.



The best advice would be to monitor surface temps of the tire face under repeatable conditions, monitor tire psi vs those temps and surface ware on the tire (shooting for even ware).
An even temp across the face of the tire will inevitably show you the best 'proper inflation' pressure for the tire and your individual car's weight ... assuming alignment/caster/camber are all within spec.
 

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Your 'factor' does not take into account that the pressure vessel is of varying volume. It only has a constant volume at rest or with no loading.
Which means that road conditions can certainly have an adverse effect of the tires, most all times in a bad way.
So you're saying only the bottom of the tire flexes - how do it know? When the bottom flexes, the rest of the tire expands slightly.

Tell you what - find yourself a real good tire gauge - one of those racing ones that is graduated in 1/4 pounds. Jack the car up so the tires are off the ground and check the pressure carefully. Then set the car down on the tires and recheck the pressure and see what the difference is. Let me know; I'll be right here waiting.
 

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So you're saying only the bottom of the tire flexes - how do it know? When the bottom flexes, the rest of the tire expands slightly.
Not necessarily. As I am sure you are aware, rubber is capable of elastic deformation. I know that race tires can stretch so much that they come off the bead if they are run at too low a pressure. You can watch videos of this happening on touring cars and the top of the tire does not contract when the bottom is elongated by cornering forces. So the volume of a tire can change.

Now, I will say that without a doubt that the change in volume is going to make absolutely zero meaningful difference to the topic at hand. Just in theory (PV=nRT) the V of the tire can change so something else has to balance it out. Chances are, it will be P. But as was stated before, the safety margin for wheels is enormous because the mode of failure is so extreme. You pull the tire a little in cornering and you aren't getting neat the margins since the tire will likely fail long before the wheel.

Personally, I think that you should keep cold inflation pressures to +/- 10% of the factory recommendations. The guys who build cars don't always get it 100% right and certainly not 100% right for each owner, but in general they are pretty bright and second guessing them on something as critical as the only points of the car that touch the ground for the sake of a couple MPG strikes me as penny-wise, pound-foolish.
 

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I'll concede your points. Under extreme conditions the tire can distort; one reason the off road boys run beadlock rims, the ability to air down and not pull the tire off the rim. I also think there is very little reason to run at the pressure listed on the sidewall, even if my F150 came that way. My replacement truck tires are 44 max and my Focus are 45 max. I will admit to boosting my fronts 2-3 psi over the door label for a little crisper handling. (F-150 is 35 R and 32 F; I run 35 all around, Focus is 34 all around; I raise the fronts to 36) Unlike a larger rear sway bar, I can undo the changes in a couple of minutes if the road is icy. In fact, I'll run the F150 at 30 R and 32 F in a bad snow storm and can go anywhere I need.
 

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w/ my magic bag
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I 2nd or 3rd that!

The trade off between accelerated shock wear, tire wear (the center of the tread will in all likelihood wear quicker), poorer braking and handling doesn't seem worth the minimal mpg increase over the long run. It could result in being an expensive experiment (even if potential tire damage from bad roads or debris is not considered).
I 2nd or 3rd that. Wow. You could alway's drive like there an egg under your foot, to you know!
 

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Your 'factor' does not take into account that the pressure vessel is of varying volume. It only has a constant volume at rest or with no loading.
Which means that road conditions can certainly have an adverse effect of the tires, most all times in a bad way.
The safety factor designed into the tire will take into account road conditions and any foreseeable abnormal conditions, even inaccurate gauges used to fill the tires.

Any volume variations are small and are of little consequence. The tire certainly changes shape but the volume remains fairly constant. The belts and cords do a good job at not allowing volume change.

I threw out 2x of a side wall rating of 44 psi just to illustrate that there is no point in worrying about single digit numbers over the side wall rating.
 

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w/ my magic bag
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spring lbs.

I'll concede your points. Under extreme conditions the tire can distort; one reason the off road boys run beadlock rims, the ability to air down and not pull the tire off the rim. I also think there is very little reason to run at the pressure listed on the sidewall, even if my F150 came that way. My replacement truck tires are 44 max and my Focus are 45 max. I will admit to boosting my fronts 2-3 psi over the door label for a little crisper handling. (F-150 is 35 R and 32 F; I run 35 all around, Focus is 34 all around; I raise the fronts to 36) Unlike a larger rear sway bar, I can undo the changes in a couple of minutes if the road is icy. In fact, I'll run the F150 at 30 R and 32 F in a bad snow storm and can go anywhere I need.
No problem w/ that, like changing spring lbs.
 
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