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Thread: Tire pressure - Hot or cold?

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    Senior Member daniel's Avatar
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    Tire pressure - Hot or cold?

    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. #2
    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.

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    Roadster 919, S 2006 Doug_G's Avatar
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    Quote Originally Posted by S-2000 Roadster View Post
    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.
    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.

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    Senior Member smorgasbord's Avatar
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    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:

    WARNING: Tire pressures should be checked using an accurate pressure gauge when cold.
    Check the pressures when the tires are cold. Be aware that it only takes 1 mile (1.5 km) of driving to warm up the tires sufficiently to affect the tire pressures. If it is necessary to check the tires when they are warm, you should expect the pressures to have increased. Do not let air out of warm tires in an attempt to match the recommended cold tire pressures.
    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:

    Always inflate your tires to the Tesla Motors recommended inflation pressure even if it is different from the maximum inflation pressure information found on the tire itself.
    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.

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    Head Moderator / Administrator doug's Avatar
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    Quote Originally Posted by smorgasbord View Post
    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.
    What he said.


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    Member Slackjaw's Avatar
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    Quote Originally Posted by daniel View Post
    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.
    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...
    Roadster 2.5 #1329 | Obsidian Black with "oooh! Shiny Wheels!" | Central NJ

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    Senior Member daniel's Avatar
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    Quote Originally Posted by Slackjaw View Post
    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...
    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.

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    Lightning Green Fairytale Dragon's Avatar
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    Quote Originally Posted by Slackjaw View Post
    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...
    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.
    Last edited by Dragon; 2011-07-13 at 06:18 AM.
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    Member Slackjaw's Avatar
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    Quote Originally Posted by Dragon View Post
    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.
    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.
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    Quote Originally Posted by Slackjaw View Post
    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...
    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.
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