Why do highly-rated shops overfill my tires?
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Thread: Why do highly-rated shops overfill my tires?

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    Why do highly-rated shops overfill my tires?

    I have a Boxster S 2013 with 20" wheels. The book says 33/33psi front and back.

    Two shops I use are both very highly regarded, one an indy mechanic and another an alignment shop I use for mounting my tires.

    On different occasions, both have set pressure at high levels, this time 38/44 which was 41/46 by the time I drove off the lot.

    Why might they do that? Is that the default pressure of a 911? Or different tire size? Is it some cool race-guy setup?

    I don't think it's an improvement for the car. Seems jittery and even more so that when my car is in the shop I drive a cushier vehicle.

    Thanks.
    2013 Boxster S acquired late Dec 2015.
    Platinum Silver w/ Black, 20" Carerra Classic wheels, SC, BOSE, PSE, SportDesign wheel.

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    Re: Why do highly-rated shops overfill my tires?

    I keep my tires spot on the pressures they are supposed to be. But every time I take it to either a Porsche dealer or Indy shop for service they always return it to me with overinflated tires, sometimes by as much as 5 lb. So, trickybit, it's not just you.
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    Re: Why do highly-rated shops overfill my tires?

    I hate when they do that. Possibly they’re just looking at the Sticker in the door jam.

    When I take my car in for service I tell them no wash and leave my tire pressure alone.
    Agate Gray 2016 Cayman GTS, 6 Speed, GTS interior.

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    Re: Why do highly-rated shops overfill my tires?

    Shops will generally overfill low-profile tires to “protect the rims”. I’m not saying this is the right thing to do, but the perception is that higher tire pressures will be more likely to prevent a dented rim after you fall into a pothole.

    It may be annoying, but they are doing it as a service to you. I personally keep mine at 33psi as the door jam specifies...

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    Re: Why do highly-rated shops overfill my tires?

    Those are the recommended pressures of my 911s, not the Spyder.
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    Re: Why do highly-rated shops overfill my tires?

    Agreed, tires usually come back from any old tire place somewhat over-inflated. Never bothered me since I am perhaps overzealous in regularly checking and setting tires to my favorite pressure. I figured the tire shops over-inflated as a favor to me so I could simply release air until I got to my desired pressure without having to turn on my compressor.

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    Re: Why do highly-rated shops overfill my tires?

    Quote Originally Posted by trickybit View Post
    this time 38/44 which was 41/46 by the time I drove off the lot.

    Why might they do that? Is that the default pressure of a 911? .
    36/44 is standard for a 911. That could be it but I've been told from different places, different dealers, different car brands, not just Porsche, its MPG. The higher the pressure, the higher the MPG.

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    Re: Why do highly-rated shops overfill my tires?

    Over-inflated tires will show signs of accelerated wear along the center of the tires, because the center is making more contact with the road than the outside edges are. This same decrease in contact surface may also cause a small drop in mpg, as the engine now has to work harder to compensate for the reduced contact patch. For best performance and tire longevity, manufacturer recommendations should be followed.
    I suspect that tire pressure checking is relegated to a dealer's least experienced and paid tech. If it were me, I'd bring this issue to the attention of the SA, and I'm certain it won't recur.

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    Re: Why do highly-rated shops overfill my tires?

    I can't even remember the last time I picked up any car from service when the pressure wasn't way over what it was when I took the car in. Doesn't matter where or who, it happens every time. Whether a dealer, an independent, a tire shop, you name it, they over inflate. I'm actually quite anal when it comes to proper/recommended pressures and check them faithfully. All my cars are driven with the tire pressure showing on the dash at all times. I've taken to telling the service businesses to NOT adjust my tire pressure any longer as it's something I take care of faithfully. Most comply but some forget and I adjust it as soon as I get home. Never understand why or how it happens but it does.

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    Re: Why do highly-rated shops overfill my tires?

    In my experience, with any service is they do not check/adjust tire pressure even if it is on their checklist of things to do. They will though over torque wheel bolts or nuts.

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    Re: Why do highly-rated shops overfill my tires?

    Thanks everybody. My conclusion is that's it's probably the 911 standard confusing them. It wasn't a side effect of mounting, and it wasn't unintentional or negligence in either case. I'm pretty sure neither of these shops are favoring mileage over performance...

    I'll bleed them down to where I want them, but not without being in reach of my compressor because I'm not confident that I've got the touch.
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    Re: Why do highly-rated shops overfill my tires?

    One other thing to keep in mind is that most tire pressure gauges are very inaccurate. Having been a master distributor for Intercomp, I have tested a number of pressure gauges and have talked with their engineers on the topic. Dial and standard digital gauges can vary +/- 5 psi. These include ones used by tire shops and very seldom will two read the same. This is why professional racing teams use calibrated digital gauges that cost hundreds of dollars and read to a hundredth of a pound. Manufacturers recommend even those are returned once a year for recalibration. I happen to have one of these gauges and take it with me whenever I have tires mounted or rotated. You’d be amazed how inaccurate the gauges tire shops use are. I always tell the installer to over inflate my tires and then I bleed them down to where I want them as soon as they are mounted. With that all said, the gauge you keep in your car or toolbox may not be as accurate as you think or as accurate as the tire shop’s. Also remember there are a number of other factors that can cause tire pressures to fluctuate after they are mounted; the car sitting in the sun vs shade, and ambient temperature change(1 psi for every 10 degree change up or down). Jus food for thought.


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    Re: Why do highly-rated shops overfill my tires?

    Interesting; I assumed there were pro-grade gauges. In my case, what I measured and saw on the dash display was consistent with what the techs said they did intentionally.

    For my own use, I've compared my few consumer-grade gauges to each other and they're in agreement, certainly within the precision that matters after you consider the variations due to ambient, radiant and go-fast temperature changes.

    Thanks for the info.
    2013 Boxster S acquired late Dec 2015.
    Platinum Silver w/ Black, 20" Carerra Classic wheels, SC, BOSE, PSE, SportDesign wheel.

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    Re: Why do highly-rated shops overfill my tires?

    I overfill mine by a couple of PSI. Found tire life lasts longer that way. When I track I rely on hot temps tire pressures and adjust accordingly. Then I get home and fill them back up a couple psi over recommended. So far with 4 track days and nearly 12k miles in 3 years my tread wear is fine but the compound is gone. The tires have no grip. Makes it fun to drive around though sliding everywhere under power especially in these super cold temps in Atlanta.

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    Re: Why do highly-rated shops overfill my tires?

    Quote Originally Posted by danschy View Post
    Agreed, tires usually come back from any old tire place somewhat over-inflated. Never bothered me since I am perhaps overzealous in regularly checking and setting tires to my favorite pressure. I figured the tire shops over-inflated as a favor to me so I could simply release air until I got to my desired pressure without having to turn on my compressor.
    This is exactly how I look at it. Next coldest morning in a few days I adjust before driving in morning, good to go.
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    Re: Why do highly-rated shops overfill my tires?

    So.... on the subject of accuracy.... How close are Porsche TPM's readings??

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    Re: Why do highly-rated shops overfill my tires?

    Here in crazy California, shops are required to make sure the tires are inflated. I suppose this is supposed to save gas.

    Both times I got new tires the tires were over. Before the second time of getting tires I asked what pressure they were going to fill and it was higher than recommended. I specifically told them the cold front and back and ... I got the car back with over-inflated tires. Well, at least it's easy to let air out of tires!

    Sounds like we need someone to lose control, crash, and sue for improper service. That'll get more attention on the problem.

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    Re: Why do highly-rated shops overfill my tires?

    My niece's Prius just came home with 2 new tires from Firestone, both set approximately 6psi high, so I bled them down to where they belong.

    Quote Originally Posted by upwinger View Post
    So.... on the subject of accuracy.... How close are Porsche TPM's readings??
    This has been a bit of a mystery for me. I have an 07 Cayman Base; put on aftermarket TPMS sensors a couple years ago when batteries died. My TPMS reads about 2psi low, but seems to be able to correctly tell me when pressure is set per spec. I only consider the TPMS accuracy suitable for alerting me to a leaky tire.

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    Re: Why do highly-rated shops overfill my tires?

    Quote Originally Posted by upwinger View Post
    So.... on the subject of accuracy.... How close are Porsche TPM's readings??

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    There's an entire thread on this issue. There's about 1 PSI difference in readings between TPM and my tire gauge.

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