# Checking Tire Pressure when the tire is warm or cold

Witnessed a recent debate about what is the appropriate temperature to check tire pressure at. The debate centered around warm or cold, there wasn't an exact temperature given by any of the parties.

What is the appropriate time to take your tire pressure and fill your tires?

When they are warm or when they are cold?

Why should tire pressure be taken at that particular time of warm or cold?

• And why should it be done that way ... Jan 8 '16 at 0:49
• Tires should be inflated to their proper pressure at ambient temperatures, whatever that happens to be for that time of year. They should not be inflated while hot from freeway driving, etc. If you have to drive some distance to get to a gas station to inflate your tires, you should check the pressure before you leave and after your car has been sitting for awhile. Inflate them to the difference, i.e. if your tires are 4 lbs low, then put 4 lbs of air in them at the gas station, even if that means going over the recommended pressure. Jan 8 '16 at 1:56
• It seems to be a debate point on some threads. I threw out the question to get everyone to pile on regarding filling tires when they are hot or cold. We'll see what happens. Jan 8 '16 at 1:57
• Oh...and don't check your tire pressure if one side of your car has been sitting in the sun for awhile. I've actually measured them, and the side sitting in a hot summer sun for hours can be 6 lbs higher than the side that isn't. Jan 8 '16 at 2:00

Cold. All OEM's door placards specify the tire pressure be set when the tires are "cold". But what is cold? From a random Honda owners manual:

Measure the air pressure when tires are cold. This means the vehicle has been parked for at least three hours, or driven less than 1 mile (1.6 km). If necessary, add or release air until the specified pressure is reached. If checked when hot, tire pressure can be as much as 4–6 psi (30–40 kPa, 0.3–0.4 kgf/cm2) higher than if checked when cold.

Note: This means ambient outside air temp, not inside a warm garage.

Low tire pressures carries far higher risk than high pressure. Since low pressure causes tire heating which leads to rapid deterioration and possible explosion. Low tires also cause poor handling due to higher flexibility in the sidewalls.

High pressure carries a shorter list of problems; The smaller and misshaped contact patch increase centerline wear and can reduce road friction.

The OEM's want the pressure on the higher end of the allowed range because of the lower risk and the pressure is going to drop naturally.

• Wouldn't a small contact area decrease hydroplaning risk rather than increasing it, since a small area would require greater pressure to cause hydroplaning? Put another way, if a smaller area made hydroplaning easier, wouldn't water skis be thinner? Jan 8 '16 at 6:33
• Been thinking a lot about this one this morning. I actually don't know the answer, just thinking out loud. The smaller patch is also more rigid (higher tire pressure) so it will tend to lift rather than deform the tire when the upward force is applied by the weight of the car pushing down and the water pushing back up. But I think I agree with the water ski analogy. Water ski's are wide because a wider ski generates more lift given a constant speed over the water. A wider patch will generate more lift. Somehow this feels wrong, but I'm not sure I see any error in it. Other thoughts? Jan 8 '16 at 13:02
• Measuring cold makes the most sense too. If you measure when the tire is at ambient (not driven recently), you get a rough baseline pressure. Figure on checking the tire pressures about once a month. Over that time temperatures only vary about 10-ish deg C, roughly. That's a shift of 3-ish PSI. Once you drive the pressure is only going to go up. So inflate the tire so it's never under inflated for the given ambient swing, and so that driving never gets the pressure over the max. To do that, measure cold, inflate to placard pressure. Great question! Jan 8 '16 at 13:22
• According to NHTSA a rough formula for calculating when a tire with a given tire pressure might hydroplane is Hydroplaning speed = 10.35 x square root of inflation pressure. If you plug in the numbers a tire inflated to 30 psi might hydroplane at 56.7 mph, 25 psi at 51.8, and 20 psi at 46.3. The URL is "nhtsa.gov/cars/rules/rulings/TPMS_FMVSS_No138/part5.6.html". So clearly low tire pressure increases hydroplaning risk. One would expect the converse to be true. Jan 8 '16 at 20:18
• @BillOer I stand corrected and have changed the high pressure hydroplaning comment. So that leaves little reason to run tires at low pressure. Jan 8 '16 at 22:07

Concerning hydroplaning (aquaplaning), higher than recommended tire pressures, and braking/steering "grip", let´s remove unnecessary complication/confusion. The factors at hand are #1 Contact surface of the tire with the pavement, #2 Tire capability to pump water through the groves, #3 Suspension behavior. The more contact surface... the better the braking/steering. You want friction.That simple!! Overinflation reduces contact surface. With proper remaining grooves, the tire will pump out water "adequately", this meaning that any and all tires will start hydroplaning to some degree after a given speed (Square root of tire pressure in Lb. x 9 = Initial hydroplaning speed MPH) is a close enough calculation, let´s be practical and not "millimetric"). Obviously, if you are driving in 4 inches of water no tire in the world is going to pump all that out. High tire pressures induce a harder and longer suspension travel and, especially with old or oil shock absorbers that loose performance as they heat up and part of the oil becomes "bubbly" due heat, there is a segment of time with no tire contact at all with the road when the tire is between the max up travel from a given "bump" on the road and rebound. The engineering departments of the manufacturers of our vehicles have tested behavior in just about every possible driving condition, from the brrrrrr of Sweeden to the Saudi deserts (the Europen ones do) so... just follow the Vehicle Manual in ALL aspects. If you want to go to extremes... under certain conditions is possible to get hydroplaning... in a completely dry road, providing air humidity and temperature are high enough. In aviation all cockpit flight crews kept that in mind. I hope I helped a bit.