How to Determine Load Capacity Requirements for Replacement Tires

Short answer: A replacement tire needs a load capacity at least equal to the original equipment tire specified for your vehicle. In practice, the replacement tire's load index should be equal to or higher than the original tire's load index, in the same tire category: P-metric, Euro-metric or LT. The tire must also be inflated to a cold inflation pressure at which it actually reaches the load it has to carry. Start with three sources: the vehicle tire placard, the original tire's sidewall, and the tire manufacturer's load-inflation tables. Together they show whether two tires on an axle can support that axle's rated load.

What Do the Numbers on a Tire Sidewall Mean?

A tire sidewall carries several kinds of numbers. Some are dimensions, some are ratios, some are pressures and loads in physical units, and some are coded performance ratings. Take a typical marking such as 225/45R17 94V XL and read it in parts.

Marking	Type of value	Plain-language meaning
225	Dimension (millimetres)	Nominal section width: the width of the tire from sidewall to sidewall, measured on a specified rim width.
45	Ratio (percent)	Aspect ratio: the sidewall height as a percentage of the section width. It is not a length on its own.
R	Construction code	Radial construction. It is not a rating.
17	Dimension (inches)	Rim diameter: the diameter of the wheel the tire fits.
94	Coded performance rating	Load index: a code for the maximum load one tire can carry. Load index 94 equals 670 kg (1,477 lb).
V	Coded performance rating	Speed symbol: the maximum speed category. V equals 240 km/h (149 mph).
XL	Load construction	Extra Load, also marked Reinforced or RF. The tire can carry more load than a standard load tire of the same size, but only when it is inflated to a higher pressure.

Many sidewalls also print a separate line such as MAX LOAD 670 KG (1477 LBS) and MAX PRESS 300 kPa (44 PSI). The maximum load is the same value that the load index encodes, written out in kilograms and pounds. The maximum pressure is the highest cold inflation pressure the tire may be inflated to. It is not the pressure you should normally run.

What Is the Tire Load Index?

The tire load index is a numerical code, usually between about 70 and 130 on passenger and light truck tires. Each code stands for the maximum load in kilograms that one tire can carry under the conditions defined by its tire standard. A higher number always means more capacity. The relationship is not linear, though: one step in load index adds roughly 15 to 50 kg depending on where you are in the scale.

Load index	Max load per tire (kg)	Max load per tire (lb)
87	545	1,201
88	560	1,235
89	580	1,279
90	600	1,323
91	615	1,356
92	630	1,389
93	650	1,433
94	670	1,477
95	690	1,521
96	710	1,565
97	730	1,609
98	750	1,653
99	775	1,709
100	800	1,764
101	825	1,819
102	850	1,874
103	875	1,929
104	900	1,984
105	925	2,039
110	1,060	2,337
113	1,150	2,535
116	1,250	2,756
118	1,320	2,910
121	1,450	3,197

The load index value is a maximum. The tire reaches it only at the inflation pressure its standard ties to that maximum. For Euro-metric passenger tires, the standard load maximum is defined at 250 kPa (about 36 psi) and the Extra Load maximum at 290 kPa (about 42 psi). At lower pressures, the tire carries less. The tire manufacturer's load-inflation table shows exactly how much less.

Single and dual load indexes on LT and commercial tires

Light truck (LT) tires and European commercial (C) tires often show two load indexes, such as 121/118 or 109/107. The first number is the capacity per tire when the tire is mounted singly. The second number is the capacity per tire when it is mounted as part of a dual pair on one side of an axle. For a 121/118 tire, that means 1,450 kg single and 1,320 kg per tire in dual fitment.

Load range on LT tires

LT tires also carry a load range letter. The letter indicates the tire's strength and the maximum pressure it is built for:

• Load Range C: 6-ply rating, maximum load reached at about 350 kPa (50 psi).
• Load Range D: 8-ply rating, maximum load reached at about 450 kPa (65 psi).
• Load Range E: 10-ply rating, maximum load reached at about 550 kPa (80 psi).

A higher load range only adds capacity when the higher pressure is actually used. At the same pressure, a Load Range E tire and a Load Range C tire of the same size carry about the same load.

Where Do You Find the Load Requirement for Your Vehicle?

The load requirement for a replacement tire comes from the vehicle, not from the tire. Three markings define it.

• Vehicle tire placard: This label is usually on the driver's door jamb, sometimes inside the fuel filler door or glovebox. It lists the original equipment tire size and the recommended cold inflation pressures for the front and rear tires. It often also shows the maximum combined weight of occupants and cargo.
• Original equipment tire specifications: The owner's manual, or the sidewall of the tires the vehicle came with, gives the original load index and speed symbol. Together these form the service description, for example 91H or 94V.
• Axle load ratings: The vehicle certification label lists the Gross Axle Weight Rating (GAWR) for the front and rear axles. GAWR is the maximum weight each axle is designed to carry.

The placard pressure is set for the original equipment tire size and load type. If the replacement tire has a different size or a different load construction, the correct pressure may differ from the placard. The load-inflation tables are used to confirm that.

How to Determine Load Capacity Requirements for a Replacement Tire

To determine replacement tire load capacity, compare the new tire's capacity with both the original tire's rating and the vehicle's axle ratings, at the pressure you will actually use.

1. Record the original tire data. Note the size, load index and speed symbol from the placard, the owner's manual or the original tires. Note whether the original tire is P-metric (the size starts with P), Euro-metric (no prefix), LT, or C.
2. Convert the original load index to a load. Use the load index table. For example, load index 91 equals 615 kg (1,356 lb) per tire.
3. Read the axle ratings. Take the front and rear GAWR from the certification label.
4. Check the replacement tire's load index. It should be equal to or higher than the original load index. If the original tire was Extra Load, the replacement should be Extra Load as well, or carry an equivalent or greater capacity.
5. Apply the category correction when needed. When a P-metric passenger tire is fitted to a light truck, SUV or van, the tire industry practice in North America is to divide its load capacity by 1.10. A P265/70R17 with load index 113 is rated 1,150 kg (2,535 lb). Divided by 1.10, that gives about 1,045 kg (2,305 lb) of usable capacity on that type of vehicle.
6. Check the axle. Multiply the per-tire capacity, at the pressure you will run, by the number of tires on the axle. Use two for a single-wheel axle, or four with the dual load index for a dual-wheel axle. The result must be at least the GAWR.
7. Set the pressure from the load-inflation table. Find the cold inflation pressure at which the replacement tire carries at least what the original tire carried at the placard pressure. Never exceed the maximum pressure molded on the sidewall.

Worked example: axle check

Assume a car with a rear GAWR of 1,300 kg and original tires with load index 94 (670 kg each). Two tires give 2 × 670 kg = 1,340 kg, which is above 1,300 kg. A replacement tire with load index 91 (615 kg) would give 2 × 615 kg = 1,230 kg, which is below the axle rating, so it is not suitable. Remember that these figures are maximum loads at the reference pressure. At a lower pressure, the actual capacity is lower, so the check is only valid at the pressure the tire will actually run.

Why Inflation Pressure Changes Load Capacity

A tire's load capacity depends on its air pressure. The compressed air, not the rubber alone, supports the vehicle. Load-inflation tables published under the tire standards list the load each tire size can carry at each cold inflation pressure. "Cold" means the pressure is measured before driving, or after the vehicle has stood for several hours.

This explains three common situations:

• Standard load versus Extra Load: At the same pressure, up to the standard load reference pressure, a Euro-metric XL tire carries roughly the same load as a standard load tire of the same size. The XL tire's extra capacity appears only as pressure rises toward its higher reference pressure.
• Switching between P-metric and Euro-metric: The two systems use different load-inflation tables. A Euro-metric tire with the same load index as the original P-metric tire may need a different pressure to carry the same load. Check the table for the specific replacement tire rather than assuming the placard pressure carries over.
• Underinflation: A tire run below the pressure its load requires loses capacity, flexes more and builds heat, even if its load index looks sufficient.

How Do Tire Size Changes Affect Load Capacity and Geometry?

Changing tire size changes two separate things: the tire's load capacity, which follows its size, construction and air volume, and its geometry, meaning sidewall height, overall diameter and circumference. Both need checking. A size that matches the original diameter closely can still carry less load, and a size with ample load capacity can still be the wrong diameter.

How to calculate sidewall height, diameter and circumference

Three simple formulas turn the size marking into approximate dimensions, using millimetres throughout:

• Sidewall height (mm) = section width (mm) × aspect ratio ÷ 100
• Overall diameter (mm) = rim diameter (in) × 25.4 + 2 × sidewall height (mm)
• Circumference (mm) = overall diameter (mm) × π (about 3.1416)

Width and aspect ratio work together. A wider tire with the same aspect ratio has a taller sidewall, because the ratio is a percentage of the width. A larger rim diameter with a lower aspect ratio can keep the overall diameter nearly unchanged; this is how plus-sizing works.

Size comparison example

Size	Sidewall height	Overall diameter	Circumference	Diameter vs 205/55R16
205/55R16	112.8 mm	631.9 mm	1,985.2 mm	Reference
225/45R17	101.3 mm	634.3 mm	1,992.7 mm	+2.4 mm (+0.4%)
215/55R16	118.3 mm	642.9 mm	2,019.7 mm	+11.0 mm (+1.7%)

In this example, 225/45R17 is almost the same overall diameter as 205/55R16, but its sidewall is about 11.5 mm shorter. A lower-profile tire often holds less air. To match the original load index, it may therefore need Extra Load construction and a higher cold inflation pressure. The 215/55R16 is about 1.7% larger in diameter, so it covers about 1.7% more distance per revolution, which affects speedometer reading and clearance.

For load purposes, the rule does not change with size: whatever the size, the replacement tire's load index, at the pressure you will use, must be at least the original requirement.

Limits of calculated geometry

The formulas give nominal geometry, not the actual measurements of a specific tire. Several factors cause real tires to differ:

• Manufacturer and model: Two tires with the same size marking can differ by several millimetres in diameter and width. Manufacturers publish actual overall diameter, section width and revolutions per kilometre or mile for each model.
• Rim width: Section width is defined on a specified measuring rim. A narrower or wider wheel changes the mounted width slightly.
• Load and wear: Under load, the tire's radius from axle to ground (loaded radius) is smaller than half its unloaded diameter. Tread wear also reduces diameter over the tire's life.

Calculated geometry is also not the same as fitment compatibility. A size can match on paper and still be unsuitable because of wheel width range, clearance to suspension and bodywork, brake clearance, or the vehicle manufacturer's approved sizes. Use the calculation to understand the difference between sizes. Use the vehicle and tire manufacturers' published data to confirm the specific fitment.

Common Mistakes When Choosing Replacement Tire Load Capacity

• Matching size but not load index: The same size is sold in standard load and Extra Load versions with different load indexes. Check the load index, not just the size.
• Using the sidewall maximum pressure as the running pressure: MAX PRESS is a limit, not a recommendation. Running pressure comes from the placard for the original tire, or from the load-inflation table for a different replacement tire.
• Assuming a higher load range adds capacity on its own: An XL tire or a higher LT load range carries more only at the higher pressure that its rating requires.
• Ignoring the P-metric correction on trucks and SUVs: A P-metric tire's printed capacity is reduced when it is used on a light truck-type vehicle.
• Confusing load index with speed symbol: In 94V, 94 is the load and V is the speed. A higher speed symbol does not mean a higher load capacity.