The 4 psi Rule on Tires: What It Is, How to Use It, and When It Applies

The 4 psi rule says that, for most passenger-car radial tires, a correctly set cold tire pressure will rise by about 4 psi (≈0.28 bar or 28 kPa) after 20–30 minutes of normal driving; a smaller rise suggests your cold pressure was too high, while a larger rise indicates it was too low. In practical terms, it’s a quick check on whether your tire pressures match your current load, speed, and conditions, supplementing (not replacing) the vehicle manufacturer’s placard recommendations.

What the 4 psi Rule Means

The rule is a field guideline used widely by drivers, tire retailers, and motoring groups: measure “cold” tire pressure, drive to warm the tires, then measure “hot” pressure. The difference tells you how much heat (and therefore flex) the tire is generating. Around a 4 psi increase for passenger vehicles generally indicates the tire is doing efficient work without overheating. Much more than that points to underinflation (excess flex and heat); much less points to overinflation (stiff running, less heat generation).

Why It Works

Tires heat up as they flex under load. Underinflated tires flex more, build more heat, and their pressure rises more from cold to hot. Overinflated tires flex less and show a smaller rise. That heat-driven pressure change, combined with basic gas-law behavior, is what the 4 psi rule reads as a proxy for “how hard the tire is working” under current conditions.

How to Use the 4 psi Rule (Step by Step)

The following steps show how to apply the rule safely and consistently, starting from the manufacturer’s recommended settings.

1. Set your tires to the vehicle placard’s cold pressure (usually found on the driver’s door jamb or fuel flap) first thing in the morning, or after the car has been parked for at least three hours and hasn’t been driven.
2. Drive for 20–30 minutes at typical road speeds, avoiding extended stop-and-go if possible. This warms the tires to a normal operating temperature.
3. Measure the “hot” pressure immediately after stopping.
4. Compare hot minus cold: about +4 psi suggests your cold setting was about right for those conditions.
5. If the rise is less than 4 psi, your cold pressure was likely too high; reduce the cold setting slightly next time the tires are cold.
6. If the rise is more than 4 psi, your cold pressure was likely too low; increase the cold setting slightly next time the tires are cold.
7. Recheck after a similar drive on another day. Make small adjustments and stay within manufacturer limits. Do not bleed air from hot tires.

Following this routine helps dial in pressures to your current use (load, speed, ambient temperature) while respecting the vehicle’s baseline recommendations.

Important Exceptions and Caveats

Because the 4 psi rule is a heuristic, not a formal standard, consider the situations below where the target rise may differ or the rule may not be suitable.

• Light-truck (LT) and some SUV/off-road tires: Many practitioners use a larger benchmark (often around a 6 psi rise or roughly a 10% increase from cold to hot).
• Very low or very high pressures: A percentage approach (~10% rise hot vs. cold) can scale better than a fixed 4 psi.
• Heavy loads, towing, or sustained high speeds: Use the higher “loaded” pressures on the placard. The 4 psi rule fine-tunes but does not supersede these settings.
• EVs and performance/low-profile tires: Heavier curb weights and stiffer sidewalls can change heat buildup. Follow manufacturer guidance and TPMS alerts first.
• Extreme ambient temperatures: Big swings in weather affect cold starting pressure. Recheck more frequently and rely on percentage rise where appropriate.
• Track use or off-roading: Specialized pressures apply; consult event, tire, or vehicle-specific recommendations rather than the 4 psi rule.

Used thoughtfully alongside the placard and tire-specific guidance, the rule is a helpful cross-check, not a replacement for official recommendations.

Safety and Best Practices

Adhering to a few simple practices will help you get accurate readings and maintain safe margins.

• Measure “cold” correctly: after the car has been parked for several hours and out of direct sun, before driving.
• Use a quality gauge and, if equipped, ensure your TPMS is calibrated per the owner’s manual.
• Never bleed hot tires to match the cold placard number; pressures drop again as they cool.
• Stay within limits: Do not exceed the tire’s sidewall maximum cold pressure; use the vehicle placard as the baseline.
• Recheck monthly and before long trips; also check after significant temperature changes (roughly 10°F/6°C can shift pressure by about 1–2 psi).

These steps reduce wear, improve handling and braking, and help prevent heat-related tire damage.

Key Numbers at a Glance

Here are quick reference points to translate and apply the rule in different units and scenarios.

• Target rise for most passenger radials: ~4 psi (≈0.28 bar ≈28 kPa) after 20–30 minutes of normal driving.
• Alternative percentage guide: about 10% hot increase over cold (useful for non-standard pressures or LT tires).
• Ambient sensitivity: approx. 1–2 psi change for each 10°F (6°C) ambient temperature shift.

Use these figures as orientation aids, then refine using your vehicle’s placard and real-world measurements.

Bottom Line

The 4 psi rule is a practical, real-world check: if your hot pressure rises by about 4 psi from cold on a normal drive, you’re likely in the right zone for current conditions. Treat it as a tuning aid alongside the vehicle placard, tire construction, load, speed, and environment—and avoid adjusting pressures while the tires are hot.

Summary

The 4 psi rule helps confirm whether your tires’ cold pressures are appropriate by comparing cold and post-drive hot readings. A roughly 4 psi rise signals a good match for passenger radials; significantly more or less suggests adjusting the cold setting. It’s a guideline—use it with the manufacturer’s placard, tire type, and conditions in mind, and never deflate hot tires.

Can I drive with a tire pressure of 4?

Much of this will come down to the individual car, but generally, the lowest tire pressure you can drive on is usually 20 psi. Only use that lowest bar for emergencies—you should not regularly drive on tires with 20 psi unless necessary. Anything below 20 psi, and you’re driving on a flat tire.

Is it bad to overinflate tires by 4 PSI?

But you shouldn’t go too much over recommended PSI. You can safely over-inflate tires by up to 10%. For example, if your tires have a recommended PSI of 35, you can go up to around 38 PSI without worry. Anything more than that is too much and not recommended.

What is the 5 PSI rule?

The 5psi rule
Note this really only applies to on-road or smooth gravel roads. Check your tyres when they’re cold and again when they’re hot (after driving on them for ½ hour or so). If the difference between cold and hot tyre pressures is around 5psi, then you’re running the right pressures.

What is the rule of 4 for tyres?

The 4psi rule
The rule is based on the generality that if a tyre is properly inflated when cold, its pressure should increase by 4psi for passenger tyres (or 6psi for light truck tyres) under normal driving conditions on bitumen. If it increases more than 4psi (or 6psi), there wasn’t enough air in the tyres.