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Discussion Starter · #1 ·
Ok, constantly I've been seeing people (not just Focus owners) consulting their car labels (on the doors or some on the gas cap) for tire pressure. Why? They didn't make the tire, why would they know how much to fill it? Is it not correct to look on the side wall for correct pressure? I mean what if they change tire brands during assembly, now the label is off. I saw too many people this last weekend with low pressure in their tires just because the CAR said so. Read the tire & they were at least 10lbs off. Am I wrong or should I begin reading my car label & deflate my tires that didn't come with the car?

thanx... that's all
 

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The tire pressure on the sidewall is the tire manufacturer's maximum load carrying capacity at that (maximum)pressure for that tire. It is the maximum pressure that that tire should be inflated to cold. The vehicle manufacturer determines what pressure(s) is/are optimum for that vehicle.
 

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Wow, are you serious??

Never, ever, set your tire pressures by what is stamped on the sidewall. As Roger said, it is the MAXIMUM you can ever put in that tire. Just like how the door sticker says the maximum weight you can carry on your car or how your aluminum ladder says what is the maximum weight you can put on it before it colapses.

The car companies don't build the tires, but they tell the tire companies what to design for. The OEM tells the tire company what tire pressure they want to use. This is based mostly on load capacity, but also accounts for ride comfort and fuel economy. The tire company then designs the tire using the characteristics that the OEM says are important. All vehicle certification (handling, durability, fuel economy) is done at the tire pressure on the door sticker. You don't just change the tire in production either, because you have to re-certify all those things, much $$. So yes, the OEM knows what tire pressure to put in.

This isn't to say that that's the tire pressure that will work best for you. I personally set mine a couple pounds higher then the sticker. Certainly when you replace your tires, you will likely get something else and then everything changes. But using the door sticker is always the best place to start.
 

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So far what you've read is pretty much on the money. The max pressure on a tire is what you should inflate it to if it's on a car that is the max weight the tire is designed for. Car manufactures know the weight of the vehicles they build and the type, size, and design of the tire they are going to use for that specfic application. Then they recommend a tire pressure that is supposed to provide the car with the best handling and wear for that specific tire size ect. This ideal tire pressure should give your tire a good footprint, over inflation will cause tire wear down the center and too little pressure will cause the tires to wear on the outsides. Either condition will screw up the handling characteristics of you car. Take a look around and find a few different makes and models of cars that have identical (make, size, style, etc)tires on them from the manufacture, look for cars of varying sizes, then look at their recommended tire pressure chart and you will see that although they all have the exact same tire on them they will each have a different tire pressure recommendation. The key word here is recommendation cause it isn't an exact science, especially if you replace your tires with a different brand, size, style, etc. Then you should experiment using the recommended tire pressure as a base line and vary it a coupla pounds either way to find what is optimal for your car.
 

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Discussion Starter · #6 ·
Ok but once again, my tires-nor rims came with my car-or even a Focus for that matter, and as far as I've seen most tire people do put a min & max on there yes no? I'm not saying set your tires to the max...like you said, that'd be pushing it bad. Mine say 45lbs max cold I've always filled them to just under 40 using the gas station style air pumps...so what of the people here that don't have stock? And lastly, I'm sure some of you have noticed but some tires of the same size use different preasures...??? Sorry, I still don't use what the car says- I'd rather trust the people that made the tires(just not go max duh)
 

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so what of the people here that don't have stock? And lastly, I'm sure some of you have noticed but some tires of the same size use different preasures...??? Sorry, I still don't use what the car says- I'd rather trust the people that made the tires(just not go max duh)
Ask the people who make the tires and they will tell you to follow your vehicle manufactures recommendation. The low and max pressures are there for reference only. The actual pressure is based on the weight of the vehicle (load limit). If you replace your tire / wheels with a different size you still need to use the manufactures recommended pressure as a base line and adjust pressures from there for optimum.
 

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You should follow the recommendations of the car manufacturer. The proper pressure in the tire is pretty much flat across different tire brands, and is really only affected by the weight the tires are carrying. Like, all 205 tires carrying 2600lbs cars should have the same pressure, it doesn't matter who made the tire.

That being said, I like to run 35psi, cause I feel the OEMs rate them a little low to soften up the ride. Going a little firmer helps handling, and milage, and doesn't seem to hurt the wear.

I went to OK Tire in Windsor once, and after having the argument with the mechanic, I went to go talk to the owner. He ALSO INSISTED that you should inflate tires to the maximum. 45 psi in my case. They WOULD NOT listen, and I was shocked the OWNER was that dumb.

I drove the car out and deflated the tires right in front of them.

I scared me to think that everybody who buys tires from them drive away with tires inflated to the maximum.
 

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I tried posting before but for some reason it didn't show. Anyway, for those who change rim size what are you suppose to follow? You obviously can't follow the recommendation. And I would think depending on going up or down in size, the tire pressure would change as well. What tirerack had told me was to go higher to 36psi since I was going from 215/45/17 to 215/35/18.
 

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If you change the rim size and go to a tire of a lower but still acceptable load index EG 87 to 86, you want to run the tire at 36 psi to get comparable carrying capacity.

Going from 215/45/17 to 215/35/18, your're going from 87 Load Index tire to 85 Load Index tire.
 

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Well, you learn something new everyday. I was unaware of this and always went by the pressure recommended on the tire and not the owner's manual.

I also thought that you would have to run a higher PSI when you go to a bigger wheel and lower profile tire since the sidewall is smaller and the tire holds a smaller volume of air. Am I incorrect in thinking this?
 

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Discussion Starter · #13 ·
wow - this got bigger than i thought it would. I understand what you're saying but I still find it hard to believe since in most occasions when I've seen someone go off of the car the tire was obviously underflated - I mean I wouldn't drive on it with the side walls the way they looked, I've had tires pop off the bead that way.
This is interesting to know- especially since I have always use the tire as a guage on my cars, my dad's (whom I learned from) on his trucks, expensive sports cars and even heavy equipment out at job sites that do not have labels. I think it's obvious when a tire is over or under inflated, you can look at tire wear and if your sidewalls look like they're gonna fall off. If what you're saying is correct the SVT says 35lbs...that didn't look safe to me and REALLY doesn't look safe on my tires. This is weird
 

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You don't always have to you would have to run a higher PSI when you go to a bigger wheel and lower profile tire since the sidewall is smaller, because often the lower profile tire is wider and so the volume of air remains to the same. You have to check the Load Indexes of the tires.
 

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If you've seen a lot of tires failures where the tire is underinflated, tire was probably tire below vehicle spec and/or slow leaking with the drive not noticing until it was too late.

In the case of heavy trucks, EG 3/4 ton trucks, the truck spec is often set at the max load capacity of the tires, so the max psi printed on the tire matches what's on the door placard.

If there is an upside of running at max psi listed on the tire, this would keep someone who never checks the air pressure from running air pressure to low
 
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