Data Source and Methodology

This calculator's methodology is based on the load inflation tables and formulas standardized by the European Tyre and Rim Technical Organisation (ETRTO) and The Tire and Rim Association, Inc. (TRA). These standards are used by vehicle and tire manufacturers globally to ensure safety and performance.

Source: ETRTO Standards Manual, Load Formulae for Passenger Car Tyres.
All calculations strictly adhere to the principle of maintaining equivalent load capacity. When you change tire sizes, the new tire must be inflated to a pressure that allows it to carry the same load as the original tire at the vehicle manufacturer's recommended pressure.

The Formula Explained

The core calculation determines the Required Load Capacity (RLC) from your original tire and then finds the pressure needed for the new tire to meet that same RLC. The load a tire can carry is not linear; it follows a specific physics-based formula.

While the full ETRTO standards involve complex tables, the principle can be modeled with the following formulas (using PSI as the unit):

$Load = Load_{max} \times \left( \frac{P_{current}}{P_{max}} \right)^{k}$

This is inverted to find the new required pressure:

$P_{new} = P_{max\_new} \times \left( \frac{RLC}{Load_{max\_new}} \right)^{\frac{1}{k}}$

Glossary of Variables

  • $RLC$ (Required Load Capacity): The load (in lbs or kg) your original tire was supporting at the manufacturer's recommended pressure. This is the target load the new tire must also support.
  • $Load_{max}$ (Max Load): The maximum load a tire can support, as defined by its Load Index. We use a standard lookup table for this.
  • $P_{current}$ (Current Pressure): The inflation pressure of the tire.
  • $P_{max}$ (Max Pressure): The reference pressure at which the tire achieves its $Load_{max}$. This is 36 PSI (2.5 Bar) for Standard Load (SL) tires and 42 PSI (2.9 Bar) for Extra Load (XL) tires.
  • $k$ (Load Exponent): A coefficient representing the non-linear relationship between pressure and load. This value is approximately 0.8 for passenger car tires.

How It Works: A Step-by-Step Example

Let's see how to calculate the new pressure for a common "plus-sizing" upgrade.

  1. Vehicle Placard (Original): A 91 SL tire with a recommended pressure of 35 PSI.
  2. New Tire: A larger 94 XL tire.

Step 1: Find the Required Load Capacity (RLC) of the original tire.

  • From ETRTO tables, Load Index 91 = $Load_{max}$ of 1356 lbs.
  • As an SL tire, $P_{max}$ is 36 PSI.
  • $RLC = 1356 \times \left( \frac{35}{36} \right)^{0.8} \approx 1319 \text{ lbs}$

Step 2: Find the new pressure for the new tire to match this RLC.

  • From ETRTO tables, Load Index 94 = $Load_{max}$ of 1477 lbs.
  • As an XL tire, $P_{max}$ is 42 PSI.
  • $P_{new} = 42 \times \left( \frac{1319}{1477} \right)^{\frac{1}{0.8}} \approx 36.6 \text{ PSI}$

Result: The new recommended pressure is 37 PSI (rounding up). Inflating this new, larger tire to the original 35 PSI would have resulted in dangerous under-inflation and reduced load capacity.

Frequently Asked Questions (FAQ)

Where do I find my vehicle's original tire pressure?

Your vehicle's original equipment (OE) recommended tire pressure is listed on a placard, or sticker, located on the driver's side door jamb, inside the glove box, or on the fuel filler door. Do NOT use the "Max Press" written on the tire's sidewall; that is the maximum pressure the tire can hold, not the recommended operating pressure for your vehicle.

What is a 'Load Index'?

The Load Index is a two or three-digit number on the tire's sidewall (e.g., in 225/45R17 91V, the Load Index is 91). It represents the maximum weight each tire is certified to carry at its maximum inflation pressure. This calculator uses it to ensure your new tires can safely support the vehicle's weight.

What is the difference between Standard Load (SL) and Extra Load (XL)?

Standard Load (SL) tires are designed to meet their max load at an inflation pressure of 36 PSI (2.5 Bar). Extra Load (XL) or Reinforced (RF) tires have a stronger construction and are designed to meet their max load at a higher pressure, typically 42 PSI (2.9 Bar). It is critical to select the correct type, as it directly impacts the load-carrying capacity at a given pressure.

What if my new pressure is lower than the old one?

This is common if your new tire has a significantly higher Load Index (LI) than your old one. A tire with a higher load capacity requires less pressure to carry the same amount of weight. However, never go below the vehicle manufacturer's minimum recommended pressure (usually not below 28-30 PSI) as it can affect handling and stability.

What if the calculator shows a "WARNING" message?

This means your new tire's maximum load capacity is less than the required load of your original setup. This is a highly dangerous situation. You should not use these new tires on your vehicle. They are not rated to safely support your vehicle's weight, which can lead to catastrophic tire failure.

Does this work for light truck (LT) tires?

No. This calculator is specifically for P-Metric and Euro-Metric (Passenger) tires that follow ETRTO standards. Light Truck (LT) tires use different load ranges (e.g., C, D, E) and different inflation tables. Using this calculator for LT tires will produce incorrect and unsafe results.

Tool developed by Ugo Candido. Automotive engineering content verified by the CalcDomain Editorial Board.
Disclaimer: This calculator is for informational purposes only. Always consult your vehicle manufacturer and a professional tire installer before making changes.
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