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Tire Pressure Changes (And Lack Thereof) Due To Changing Temperatures

Discussion in 'Model S: Driving Dynamics' started by Andyw2100, Apr 6, 2015.

  1. Andyw2100

    Andyw2100 Well-Known Member

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    I understood that I had to be careful to monitor my tire pressures as the temperature changed. Early in the winter I had initially inflated the tires to 40 PSI (I believe--perhaps it was 42) and then a few weeks ago, when the temperature outside was in the 20s, I increased that to 46 PSI. I just checked the tire pressures again, since the temperature today is 55 F. I was expecting to have to bleed some air out of the tires (a question on that follows), but was surprised to find all four tires still at exactly 46.0 PSI, precisely what I had filled them to roughly a month ago, when the temperature was quite a bit cooler. The temperature in my garage is probably cooler than 55, and may also have been warmer than in the 20s a month ago, but even so, shouldn't I have seen some increase in tire pressure due to the increased temperature?

    As for my question on bleeding air from the tires if I had needed to, I realized that I probably don't have a good tool to do that with. Am I right to assume that I should probably just buy an inexpensive tire pressure gauge with a bleed valve, for example, something like this? Amazon.com: Accu-Gage S60XA (5-60 PSI) Swivel Angle Chuck Dial Tire Pressure Gauge with Bleed Valve: Automotive

    I'm using a digital tire pressure gauge that has no bleed valve to check the pressure, and a Craftsmen cordless inflator to fill the tires, but I don't think there's a good way to bleed the tires using that either.

    Thanks.
     
  2. dhanson865

    dhanson865 Active Member

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    #2 dhanson865, Apr 6, 2015
    Last edited: Apr 6, 2015
    buy a gauge that goes to 100 PSI, gauges are most accurate between 40% and 60% of max and a 60 PSI max gauge is too low range for measuring 46 PSI accurately. Keep that in mind for your digital gauge and whatever gauge is on the pump.

    If you need a gauge with bleed cheap maybe get Amazon.com: Accu-Gage H100X PSI Dial Tire Gauge: Sports Outdoors

    Compare the PSI between multiple tools, and pick one you trust (if two match or if one tool is constantly in the middle of three readings, of course you'll have to have at least 2 decent gauges to have a chance at figuring which ones are off). Don't assume any new gauge made it from the factory to you without being damaged. Just because it is new or looks nice doesn't mean the reading is accurate.

    What I do is pump to a few PSI above my desired PSI then let the car sit a few hours and bleed to even to the desired PSI using my trusted gauge.
     
  3. jerry33

    jerry33 S85 - VIN:P05130 - 3/2/13

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    A good 60 psi gauge will be accurate. Digital doesn't mean accurate, an accurate digital gauge will be about twice as expensive. One like this will do the job nicely.
     
  4. Andyw2100

    Andyw2100 Well-Known Member

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    Thanks for the responses.

    The digital gauge I've been using is actually this one: Amazon.com: TEKTON 5941 Digital Tire Gauge, 100 PSI: Home Improvement

    I bought that after realizing that the Craftsmen inflator, which has a gauge, couldn't be trusted. That gauge reads one figure when checking the pressure, but then when inflating it immediately increases the reading by about 1.3 PSI. So using that piece of equipment, if I wanted to inflate to 46 PSI, I'd have to inflate until the unit read 47.3 PSI, and then when I stop inflating it would read 46 PSI. I actually sent the first one of these back because of this behavior, after checking with Sears, only to learn, after receiving a second one that behaved the same way and then checking here, that this is how they all work.

    As it turns out, I think even the lower reading from the Craftsman is still high, as the digital gauge I bought consistently reads below that by about a pound.

    When I purchase a better gauge with a bleed function I guess I'll be able to tell which numbers have been closest. But my money is on the digital gauge, and not the Craftsman inflator.

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    So you seem to be suggesting Longacre as a quality product, Jerry. The styles you linked to, though, would appear to be for use with air compressors. Is it safe to assume you'd suggest something like this as a quality gauge that would meet my needs?
    http://www.longacreracing.com/products.aspx?itemid=1716&prodid=7313&pagetitle=Basic+2%22+GID+Tire+Gauge+0-60+by+1+lb
     
  5. anticitizen13.7

    anticitizen13.7 Enemy of the Status Quo

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    I own both the 60 and 100 Accu-Gauge. I prefer the 100 model, since it can comfortably measure the pressure of an emergency spare tire, which is typically nominal at 60 PSI.

    I like the old school brass mechanical gauges because they don't need batteries.
     
  6. dhanson865

    dhanson865 Active Member

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    The OP linked to a $10 tire gauge. Sure if you get into $70 and higher gauges you can get accuracy that way.

    But if you look at the PDF specs for the cheaper analog gauges many specify one level of accuracy in the 40-60% range and another less accurate rating for the outer ranges. If you are going to buy at the low end of $5 to $30 gauges there is a strong reason to buy a 100 psi gauge for modern tires with max PSIs like 44, 51, or 60.

    They also warn you that a tire inflated over the PSI max of the gauge can permanently damage the gauge. Some spare tires are rated to 80 psi.

    So yeah, if you want to buy an expensive gauge go for it. I'm just giving advice for the cheap end of the spectrum.
     
  7. eye.surgeon

    eye.surgeon Member

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    tire pressures usually vary about 1 lb per 10 degrees f ambient temp change. Keep in mind if you park in and check your pressures in your garage it will mitigate a lot of the changes in tire pressure you experience as you drive outside. Riding motorbikes I check tire pressures fanatically. A lot of riders swear by this gauge:

    Amazon.com: BMW Electronic Tire Pressure Gauge: Automotive

    ignore the BMW branding. It's a high quality digital gauge that goes to 100 PSI.
     
  8. Andyw2100

    Andyw2100 Well-Known Member

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    That looks like a fine gauge, but it doesn't look like it provides a way to bleed air out of the tires, which is what I'm looking for in my next purchase. I currently have two ways to digitally measure the pressure in the tires, but neither can bleed air out of them.

    I'm thinking I'll probably go with this one, based on Jerry33's recommendation above:

    Amazon.com: Longacre Standard 2 GID Tire Gauge 0-60 by 1 lb - 52004: Automotive

    It's made by the same company as the one Jerry recommended.

    And I'm not disregarding your posts, dhanson865, but I'm not planning on checking air pressure in spares, so I'm not worried about damaging the gauge, and I'm hoping that this gauge is a high enough quality that there's no need to get a 100 PSI gauge just to improve the accuracy in the range I'm likely to be concerned about.
     
  9. mspohr

    mspohr Active Member

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    Tesla now recommends 50 psi inflation. You should always measure pressure when the tires are cold in the morning. You should never bleed air out of a hot tire. Tire pressure will increase as ambient temperature increases, when driven or sitting in the sun. You should rarely need to bleed air out of a tire. Any small tool that can depress the valve will work.
    Here's a good article about inflation:
    http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=73
     
  10. jerry33

    jerry33 S85 - VIN:P05130 - 3/2/13

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    It's what I use. I've had good success with them. The ones I linked to are the easiest to use (and it's really worthwhile getting a compressor because you can use it for a number of other tasks). But they have equivalent ones that are just gauges.

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    100% correct.

    - - - Updated - - -

    And are half the price.

    And yes, you wouldn't use a 60 psi on a temporary spare. The main issue I have with 100 psi gauges is that unless you get one that has a very large dial (5" or 6") it's hard to read the 1 psi increments. A 3" 60 psi gauge is easy (and probably harder to drop). Generally 100+ psi gauges are for truck tires (big trucks) and within 5 psi is close enough. They are easy to read at 5 psi increments--not so much at 1 psi.
     
  11. CHG-ON

    CHG-ON Still in love after all these miles

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    Can you clarify your comment about 50 psi? I am running my P85+ at the recommended 38/40. I actually used to always run my tire 2# higher. Why? I have been doing it so long that now I actually forget! Safety margin I guess. But Tesla made such a point to me about having it right on the mark that I am now following their advice. I will say that while my Michelins wore out in 9K mi (better than some), the wear was uneven (more on the inside) but not as extreme as some photos I have seen.

    Thanks.

     
  12. mspohr

    mspohr Active Member

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    There's a thread devoted to this.
    P85D 19 tire pressure recommendation is now 50psi
    Looks like they now recommend 50 psi for all tires on all models.
     
  13. jerry33

    jerry33 S85 - VIN:P05130 - 3/2/13

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    The inside wear is because of improper toe (aided by the negative camber).

    Any tire pressure recommendation is based on a set of assumptions. Change any of those assumptions and the pressure needs to be altered to accommodate the changes. There are several assumptions, but the most important two for a Model S driver (in my opinion) are:

    1. Ambient temperature. Every tire pressure recommendation table uses an ambient temperature of 18 (65F). If the temperature is warmer, then the pressure needs to be increased. At 38 (100F) it takes 4 psi to compensate for a recommended pressure of 45 psi (3 psi for a recommended pressure of 35 psi).

    2. Laziness factor. Every tire pressure recommendation table is based on a pressure check/adjust before each driving day. If you don't check every day (99.999% of folks) or have some telemetry that continually tells you the exact pressure, you need to add some pressure to compensate. In general, you want to be sure that the tires are never below the vehicle placard pressure. (Note: this is for normal road use, not off-road or track use both have different criteria, though I doubt I'll see many Model S competing in the Baja run.)

    Also there are:

    3. Load. This is mainly for vacation trips. It's never a bad idea to add a few psi when you're loaded up with passengers and luggage.

    4. Speed. It appears that Tesla has already taken care of this, so no adjustment should be necessary.

    So at the end of the day, what you do is pick a pressure and then watch the tires to see how they are wearing--then increase (usually) the pressure until you get the results you want. Of course, pressures won't compensate for alignment, but that's another subject.
     
  14. dhanson865

    dhanson865 Active Member

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    I've been using 100psi gauges ever since I got my 2005 Prius. I'm pretty sure it isn't a big truck. I guarantee you I'm checking to 1 PSI marks.

    the gauge I'm using as a double check is the Amazon.com: EZ - Air Tire Gauge: Automotive but I've found it agrees with the slime digital on the pump to within a half a PSI so I'm OK with trusting the pump gauge also.

    The 100 psi gauges also help in that the 40-50 psi range is in the center of the gauge instead of most of the way to one end so it's easier to read in that sense.
     
  15. Andyw2100

    Andyw2100 Well-Known Member

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    I wound up going with this one:

    Amazon.com: Deluxe 2 ½ GID Tire Gauge 0-60 by ½ lb - 52003: Automotive

    It cost more than I originally intended to spend, but I figure I won't be buying any more pressure gauges for a while. Once I had decided to go with the almost $30 Longacre anyway, I figured I may as well spend the extra money for this one, which adds the half-pound increments, the larger dial, the swivel chuck, the high-flow release button, and the "hold until release" pressure reading over the less expensive Longacre gauge. I think I should be in good shape now.

    Of course now I've got to add some pressure to my tires. I had already noticed that the ride seemed rougher than I liked when I increased the pressure last time. I can only imagine how harsh I'm going to find the ride on some of the poorly paved roads I travel once I've inflated the tires to 50 PSI, which I'll do Friday morning. It will certainly be interesting. On the upside, it may improve my efficiency a bit, I guess.
     
  16. gzerninplatz

    gzerninplatz Member

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    I put Fobotire pressure monitors on my car....why not always know your pressure without having to bleed air - or even go outside - to do it? And they correct for altitude/barometric pressure, which standard gauges do not, so you don't get a skewed reading. And you can check anytime you like just from your phone...

    To reset pressure, just overinflate a tad, then loosen the Fobo monitor cap and retighten until your phone display says pressure is right.
     
  17. jerry33

    jerry33 S85 - VIN:P05130 - 3/2/13

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    1. Overinflation means that you have set the cold tire pressure to higher than the number printed on the sidewall of the tire.
    2. I'd check the Fobo on a regular basis with a quality gauge.
     

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