Reader Question: Tire Pressure?

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Here’s the latest reader question, along with my reply!

Charles asks: I apologize if you’ve already addressed this (searched old reader questions as best I could to no avail) but could you speak to proper tire inflation? I see the number on the sidewall, but since the pressure changes with air temperature and when the tire warms up while driving… what’s a boy to do? PS: Thanks for all of your consistently awesome content!

My reply: It’s generally true that tire pressure specifications are premised on cold inflation pressures. This being to allow for increased pressure as the tire heats up. If your car has digital display tire pressure readouts you can watch the pressure increase a bit as you drive. The cold inflation specs make allowance for this increase (there may also be a hot inflation number, which of course will usually be lower than the cold inflation pressure recommendation).

There is also sometimes a difference between the pressure recommended by the car manufacturer, for the factory-equipped tires – and the pressure recommended by the manufacturer of tires you purchase later on, which may be of a different type (e.g., high-performance vs. all-season vs. snow).

There’s some wiggle room here – but so long as you don’t over-inflate or under-inflate the tires, all should be well!

. . .

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  1. In west Texas I set the wife’s FWD to 42.5 front and 30 rear and get perfect tire wear. And when you get into cold winter, you need to reset all four. I set the tire pressure on a vehicle pretty often.

    I felt pretty good till I saw the pic of Greta Thunturd. Shilling for a ride on a big airliner so she can get on her rant and say over and over “How dare you, How dare you”. With all the physical and mental problems she has, I wonder if a big spliff wouldn’t help her. That and a few shots of 100 proof bourbon…….speaking of which…….

    • I figure the tires will warm up to “normal” pressure after a few miles of driving. I don’t mess with them much as long as they pass the kick test, unless we are heading out on a long highway trip or maybe in the spring after it’s been sitting a long time.

      Our fwd car sill plate states 35/35 for the original low profile tires/wheels. I’m thinking they don’t need that much with the smaller wheels and taller sidewalls (same OD). Maybe 30-32 ??? I’ve never done a chalk test.

      The jeep probably doesn’t even need 30 psi since it’s so light and the tires are fairly big. OTOH, the LT235/75r15 need about 45 just to hold up the suburban and not look half flat.

    • A spliff the size of New Jersey just might mellow out that piece of Swedish foul work! Don’t waste good Kentucky firewater on her, though, she’s not good enough. Maybe some of that Lutefisk, or Surstromming fermented fish stuff, would match her foul attitude!

  2. Is “cold” a 60 degree summer morning or 20 below in the winter ???? Ha!

    I’ve seen at least 5 psi change seasonally. I tend to just let them start out a little low in cold weather. They’ll warm up as you drive.

    We have a neighbor with a Subaru and she was always calling me over to check her tire pressure because of the TPS warning. The tires were nearly always just fine. She got a newer one (sigh…) and I don’t know if it works better or she’s just getting someone else to check them now.

    My boot toe works pretty good most of the time 😉

    • It’s easy to estimate how temperature affects pressure. There is only one thing you need to understand, and only a couple of numbers you need to remember.

      The thing you need to understand is very simple: pressure varies directly and proportionately with temperature, but only when both temperature and pressure are expressed with respect to absolute zero.

      The temperature we call 0°F is pretty much arbitrary; it was defined a very long time ago as the lowest temperature achievable by mixing ice with salt (which is why salt does not melt road ice below 0°F). Absolute zero temperature – which is the coldest temperature possible and is defined as the temperature at which all atomic/molecular motion/vibration ceases – is -460°F. This is cumbersome, so there is a measure of temperature called the Rankine scale, which is just like the familiar Fahrenheit scale except it is zeroed at absolute zero. So -460°F = 0°R, 0°F = 460°R, 40°F = 500°R, and 90°F = 550°R.

      So the temperatures we will likely encounter in everyday life range from about 450°R to 550°R. For a rough engineering estimate, call it 500°R.

      Now for pressure. The pressure readings we are accustomed to are gauge pressure. Gauge pressure is the pressure above and beyond the atmospheric pressure all around us. When you slap a pressure gauge on a tire and it reads 35psi, it means that the pressure in the tire is 35psi greater than atmospheric pressure, or 35psig (g=gauge). But atmospheric pressure at sea level is about 15psi, so the total or absolute pressure in the tire is actually 35+15=50psia (a=absolute). 50psia is a good rough engineering estimate for tire pressure (for cars, not heavy trucks).

      Those are the two numbers you need to remember. Temperature is roughly 500°R, tire pressure is roughly 50psig.

      So let’s estimate how much tire pressure will change with temperature. Let’s say the temperature falls from 70°F to 20°F. That’s a drop of 50°F and also 50°R, which is a 10% drop in absolute temperature (50/500, roughly). Pressure will also drop by about 10%, and that is 10% of 50psia or 5 psi. That’s exactly the observation offered by Anonymous.

      But let’s say we are looking at a more serious temperature change: 90°F to -10°F. That’s a 100 degree temperature drop, or 20% (100/500). That will cause a 20% drop in absolute pressure, or 10psi (from 50 psia to 40 psia, equivalent to 35psig to 25 psig). That’s a real problem. If you are driving a truck with the tire pressure at 85psig (100psia), that 100 degree temperature drop will lower the tire pressure by 20psi, to 65psig. That’s a really big problem if you are loaded heavy.

      Fun with physical chemistry and a big glass of bourbon.

      • This and then the bourbon. Boyle’s law enters in there so the warmer the air going into the tire the higher the pressure. 100 degree air going into the tire will show higher pressure more quickly but as the temp drops the pressure will drop even more than if you were using 60 degree air. And the higher the pressure, the greater the temperature in the vessel.

  3. The factory recommended pressure should be on the door jamb of the driver’s side; it can also be found in the owner’s manual. For most modern cars, if you don’t know the pressure, you can go with 32 psi and be close enough to the factory recommended pressure that you’ll be safe.

  4. My view is that the outside edges tend to wear first from cornering. Hence I overinflate my tires by a little bit near the end of their lifetime, to put extra wear in the middle. YMMV, of course

  5. Any moisture in the tire will cause major fluctuation in tire pressure. When I park outside overnight this time of year I’ll usually have a TPS alarm on the Cherokee in the morning. After few miles it disappears. The main reason to get nitrogen instead of plain old air in the tires is because it is dry, or at least should be. I had N2 put in these tires but that was a few years ago and topping up with my shop air probably added water.

    Probably time to buy a desiccant bottle…

    • Ready – Are you the same “ReadyKilowatt” on Mish’s blog? I’ve posted a couple times in response to his “Leaving Chicago” for Utah articles, but they seem to disappear – I guess it’s because i recommend he move to New York or Hawaii instead, which is more closely aligned with his gun control beliefs (from an article of his a while back).

      Oh, and not to completely hijack this article, if you’ve got low profile tires you’ll need to check the pressure at season changes. I’ve had the TPS warnings come on after a (relatively) cold night here in south central Texas.


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