Vehicle safety and performance rely heavily on properly inflated tires. Drivers often wonder how environmental factors, particularly altitude, influence tire pressure. A common question for those traveling to higher elevations is whether the air pressure inside their tires will increase. The answer involves understanding the effects of the surrounding atmosphere and the physics of measurement.
The Relationship Between Altitude and Atmospheric Pressure
Air pressure, also called barometric pressure, is the force exerted by the weight of air molecules above a given point. At sea level, this pressure averages about 14.7 pounds per square inch (psi). Air is densest at the Earth’s surface because gravity pulls molecules downward, compressing the lower layers.
As a vehicle gains elevation, the amount of air mass remaining overhead significantly decreases. Consequently, barometric pressure decreases rapidly as altitude increases.
For example, a location at 18,000 feet has approximately half the atmospheric pressure of sea level. This reduction in external pressure is the foundational physical change that influences tire readings. This environmental change does not affect the actual amount of air sealed inside the tire.
How Atmospheric Changes Affect Tire Gauge Readings
Standard tire gauges do not measure the total or absolute pressure inside the tire. They measure gauge pressure, which is the difference between the internal tire pressure and the pressure of the surrounding atmosphere. The tire’s internal absolute pressure remains constant during ascent, assuming the temperature does not change.
When a car travels to a higher altitude, the external atmospheric pressure pushing on the tire’s exterior decreases. Since the gauge calculates its reading by subtracting this lower external pressure from the constant internal pressure, the resulting gauge pressure reading appears higher. The gauge reports an increase because the atmospheric counter-force is weaker.
For a substantial climb, such as driving to an elevation of 5,000 feet, the gauge reading may increase by about 2 to 3 psi. This confirms that the reading on the tire gauge does increase with altitude. This change is purely an effect of the measurement mechanism reacting to the less dense air outside the tire.
The Dominant Factor: Temperature’s Role in Tire Pressure
While the gauge pressure increase due to altitude is a measurable physical phenomenon, temperature plays a much larger role in real-world pressure fluctuations. Air is a gas governed by the Ideal Gas Law, which states that pressure is directly proportional to temperature when volume is constant. For a sealed tire, a change in temperature directly changes the pressure.
Tire pressure changes by approximately one psi for every 10-degree Fahrenheit change in ambient temperature. Since higher altitudes are often associated with significantly lower temperatures, this causes the actual absolute pressure inside the tire to drop. A 40-degree temperature drop, common during major elevation changes, would cause a pressure decrease of about 4 psi.
This substantial pressure drop from cooling often masks or offsets the small 2 to 3 psi increase in gauge reading caused by atmospheric pressure changes. The mechanical change from colder air contracting inside the tire has a greater practical impact than the atmospheric effect. Drivers often notice a net decrease in pressure when reaching a cold, high-altitude destination.
Practical Tips for Driving in High-Altitude Areas
Drivers should always check tire pressure when the tires are “cold,” meaning they have not been driven for at least three hours or more than one mile. This practice eliminates the temporary pressure increase caused by friction-induced heat. The proper pressure is listed on the placard inside the driver’s side door jamb, not the maximum pressure stamped on the tire sidewall.
If moving to a permanently higher altitude, adjust tire pressure to the manufacturer’s recommended level once the tires have cooled upon arrival. For a temporary trip, minor gauge pressure fluctuations of 2 to 3 psi are usually within the tire’s tolerance and do not require immediate correction. Modern Tire Pressure Monitoring Systems (TPMS) alert the driver only when pressure drops significantly below a safe threshold, which is typically due to temperature changes rather than altitude alone.