TMC is an independent, primarily volunteer organization that relies on ad revenue to cover its operating costs. Please consider whitelisting TMC on your ad blocker or making a Paypal contribution here: paypal.me/SupportTMC

Tire pressure - Hot or cold?

Discussion in 'Roadster: Technical' started by daniel, Jul 12, 2011.

  1. daniel

    daniel Active Member

    Joined:
    May 7, 2009
    Messages:
    1,131
    Location:
    Spokane, WA
    My customer advocate told me I should check my tire pressure hot, after the car has been driven for a while, but with other cars I've always been told to check tire pressure cold. Which is it? Is the Roadster different from every other car I've driven? As far as checking tire pressure goes, that is. In every other respect it is very different as I've never had a sports car before.
     
  2. S-2000 Roadster

    Joined:
    Apr 10, 2009
    Messages:
    636
    I think it depends upon whether you're close to the limit on the tires.

    Based on advice that I received from a tire dealer back when I was a teenager, I've been running my tires at or above 40 psi on every car. Most tires have a limit close to that, and many of the specific brands and models that I've run have about 44 psi as the maximum, so I always make sure to check the pressure when the tires are hot. If I didn't then I would be risking a blowout when the heated tire exceeds the maximum pressure. I've seen as much as +5 psi going from cold to hot, with 70 mph freeway speeds being responsible for the largest change.

    Since the Tesla Roadster defaults to 40 psi in the rear, I assume that it's a really good idea to check the pressure after you've driven at freeway speeds for 20 minutes to a half hour. I just looked at the maximum psi for the rear tires that Tesla Motors provides on standard vehicles, and their max is 40 psi. If you read the label directly on the tire itself, it says you should never inflate above 40 psi. I'm actually rather surprised that they recommend running the tires at their maximum. Then again, I've probably run a fair number of 44 psi tires at 44 psi for years, and never had a problem.

    In contrast, many of the other cars I've owned have recommendations in the range of 28 psi, probably for safety and comfort. At those low pressures I would be inclined to check the pressure cold, because the increase due to heating would be beneficial.

    The advantage of running high pressure is less wear, more even wear and better mileage due to lower rolling resistance. The disadvantage is a harsher ride and less traction. On ice or in snow or rain you want less pressure, so as to increase traction.

    Note that the advice I received was that this tire dealer had never had a tire come back with wear patterns indicating over-inflation. Basically, every problem was due to under-inflation and subsequent uneven wear. As a result, he started recommending 40 psi to every customer and never had a customer come back with a problem.

    YMMV.
     
  3. Doug_G

    Doug_G Lead Moderator

    Joined:
    Apr 2, 2010
    Messages:
    15,765
    Location:
    Ottawa, Canada
    Actually, the number on the side of the tire is the maximum rated cold inflation pressure. The effects of heating are already taken into account.
     
  4. smorgasbord

    smorgasbord Active Member

    Joined:
    Jun 3, 2011
    Messages:
    2,379
    Location:
    SF Bay Area
    Cold, cold, cold, cold, cold. Check your tire pressure when the tires are cold.

    That's cold, as in C-O-L-D, as in not hot, but chilly, not driven in a while, not warmed up, not heated, but cold, as in the car hasn't moved in a while.

    Check out page 10-9 of your owner's manual:

    Every tire manufacturer, every car manufacturer says to check tire pressure when the tires are cold.


    As far as inflating to the maximum printed on the tire - don't do it! As the owner's manual states:

    The number printed on the tire is the maximum safe tire pressure for the tire (when cold). It's not the recommended tire pressure for your vehicle, which is in your owner's manual and on the side panel of the driver's door.

    Overinflation makes the ride harsher, makes it more likely that potholes and such will damage the tire, can lead to premature wear in the center of the tire. While steering will feel crisper, traction on the road will generally be reduced when the tires are overinflated. Maybe that's better than underinflation, but it's not better than proper inflation.

    Of course, Tesla has given us a nice pressure readout on the VDS. While it's not super-accurate, it's a good guide to know that at least your tires aren't way under or over inflated, which is what you want to avoid.
     
  5. doug

    doug Administrator / Head Moderator

    Joined:
    Nov 28, 2006
    Messages:
    15,876
    Location:
    Stanford, California
    What he said.

    techcenter-tireinflation.jpg
     
  6. Slackjaw

    Slackjaw Member

    Joined:
    Apr 12, 2011
    Messages:
    354
    Location:
    Central NJ
    Related question: Our tire pressure monitors show nothing unless the car has been driven for 10+ minutes. Is there a trick to get tire pressures on-screen before you leave your garage? Thanks...
     
  7. daniel

    daniel Active Member

    Joined:
    May 7, 2009
    Messages:
    1,131
    Location:
    Spokane, WA
    Actually, I checked my tire pressure while sitting in the car talking to my customer advocate about an unrelated issue. I had not driven the car since the day before, and I got read-outs on the monitor. (I had to have the hand brake on to open the tire pressure screen.) The customer advocate told me what you said: that there would be no read-out until I'd driven the car. But there was. 2.5 non-sport vin 1117.

    (Just saw this in the manual: "Note: When you turn on the vehicle, you may experience a delay of up to 8 minutes before tire pressures are displayed. To reduce the length of this delay, drive the vehicle for a short distance" I had had the car on for a while dealing with an unrelated issue.)

    Anyway, I will inflate the tires cold to the recommended pressures. Thanks everyone. I feel kind of stupid for not looking in the manual before asking. And next time I talk to the c.a. I'll tell her that her advice is contradicted in the manual.
     
  8. Dragon

    Dragon Lightning Green Fairytale

    Joined:
    Dec 6, 2010
    Messages:
    450
    Location:
    Italy
    #8 Dragon, Jul 13, 2011
    Last edited: Jul 13, 2011
    Are you sure? I checked it only twice and I think I've done it before driving. Just putting the car in park mode should work as well?
    Had a PSI of 28 on both fronts and 38 on both rears.
    I checked it again yesterday AFTER driving and got 32 PSI on both fronts and 42 on the rear left, 43 on the rear right.
     
  9. Slackjaw

    Slackjaw Member

    Joined:
    Apr 12, 2011
    Messages:
    354
    Location:
    Central NJ
    I tested it thoroughly when we got the car last month; the indicators always show "--" for all four tires when you first sit in the car (with the parking brake on). This is in line with what daniel expected but not what he got. Our TPMS showed a "fault" at one point and needed to be recalibrated by a Tesla Ranger but this behaviour did not change.
     
  10. tennis_trs

    tennis_trs 2010 2.0 Roadster Sport

    Joined:
    Aug 25, 2009
    Messages:
    302
    From comments in other threads (and here) it seems like different cars behave differently, but I don't think it is a model (1.5, 2.0, 2.5) issue. My 2.0 Sport definitely takes some time (several minutes) after startup to display; I'm not sure if it will happen before I drive or not: I'm not very patient and my memory is terrible.
     
  11. Tommy

    Tommy Member

    Joined:
    Mar 3, 2010
    Messages:
    880
    Location:
    The great OC
    After contacting the tire manufacturer to verify their recommendation, if it conflicts with your customer advocate's advise, I would politely insist he/she provide his/her reason for the contradictory recommendation. Personally, I would follow the tire manufacturer's recommendation no matter what Tesla says: The tire manufacturer is ultimately the one who warrants the tire against defects; makes no sense to go against their recommendation.
     
  12. Doug_G

    Doug_G Lead Moderator

    Joined:
    Apr 2, 2010
    Messages:
    15,765
    Location:
    Ottawa, Canada
    There may be a wheel position where the reception is poor. I believe that is why they recommend the car be moving.
     
  13. vfx

    vfx Well-Known Member

    Joined:
    Aug 18, 2006
    Messages:
    14,792
    Location:
    CA CA
    Sometimes it works morning sometime I have to drive forward 20 feet.
     
  14. Dragon

    Dragon Lightning Green Fairytale

    Joined:
    Dec 6, 2010
    Messages:
    450
    Location:
    Italy
    #14 Dragon, Jul 13, 2011
    Last edited: Jul 13, 2011
    I'm going to the garage right now and check this.

    edit: Back. It took 6 min. 11 sec. to show me the pressures. Had the car in park mode. 21° tire temperature.
     
  15. S-2000 Roadster

    Joined:
    Apr 10, 2009
    Messages:
    636
    There's nothing wrong with your advice, but I want to point out that my brand new Roadster 2.5 recommends 40 psi for the rears and the stock tires have a maximum rating of 40 psi. In other words, in this particular case the recommended is the maximum.

    Aside: Ignore what I said about heating up the tires. I was thinking back to other cars where the recommended pressure is 28 psi and the maximum pressure is 44 psi. With such a wide window of options, I chose to go with 4 to 5 psi below maximum, but still well above the recommended pressure. In the specific case of the Tesla Roadster, there is no latitude at all.
     
  16. S-2000 Roadster

    Joined:
    Apr 10, 2009
    Messages:
    636
    I'm fairly certain that the images shown are for non-belted tires. Nearly all modern tires are steel-belted, and certainly the tires provided stock by Tesla. Belted tires have different wear patterns when under-inflated or over-inflated, because the steel belts prevent the rubber from simply expanding like a balloon. I think the wear patterns are actually opposite.

    In any case, I've never seen my tires show excessive inner tread wear, even though I exceed the vehicle's recommended pressure. It's a bit of a moot point, though, since I can't exceed Tesla's recommendation without exceeding the tire's maximum rating.
     
  17. strider

    strider Active Member

    Joined:
    Oct 20, 2010
    Messages:
    2,898
    Location:
    NE Oklahoma
    The wear patterns are the opposite from the picture for RUN FLAT tires due to the additional ribbing inside the tire to provide support without any air pressure.
     
  18. daniel

    daniel Active Member

    Joined:
    May 7, 2009
    Messages:
    1,131
    Location:
    Spokane, WA
    I looked in the manual (after asking the question) and on the subject of the pressure monitor, it says that when first turned on the car will take a while before showing the pressure, but that driving for a bit will speed up the process. I went out and turned on the car, and sure enough, the pressure was not shown. The other day I had been in the car talking to the c.a. about another issue, so that the car had been on for a while when I first went to the tire pressure screen.
     
  19. daniel

    daniel Active Member

    Joined:
    May 7, 2009
    Messages:
    1,131
    Location:
    Spokane, WA
    I checked mine while inflating to the recommended 30 psi front, 40 psi rear. What my OEM stock tires say is: Don't inflate above 40 psi to seat the bead. Maximum pressure 44 psi.
     

    Attached Files:

  20. S-2000 Roadster

    Joined:
    Apr 10, 2009
    Messages:
    636
    Thanks.

    My flashlight must not have caught the second part of that in my dark garage. Now I feel better about 40 psi recommended out of 44 maximum, and it also bodes well that I was running this pressure on all of my other vehicles before Tesla made it apparent that it's way more energy-efficient this way.
     

Share This Page