Passenger vs Light-Truck Tires: Comparing Load Capacity When Replacing Tires

Short answer: A passenger (P-metric) tire and a light-truck (LT-metric) tire can share the same size numbers, but they do not carry the same load. Most passenger tires reach their maximum rated load at about 35 psi (240 kPa) for standard load or 41 psi (280 kPa) for extra load. A light-truck tire reaches its higher maximum load only at its higher load-range pressure, such as 80 psi (550 kPa) for Load Range E. A P-metric tire on a light truck or SUV must also be derated by dividing its load by 1.10. At the same moderate pressure, an LT tire can carry less than a comparable P-metric tire. So comparing load index numbers alone is not enough. You must compare each tire's load at the pressure you will actually run against the load the vehicle requires per tire.

What do the numbers on the sidewall mean?

A tire size marking combines three kinds of information: dimensions (width and rim diameter), a ratio (aspect ratio), and performance ratings (load index, speed rating, and load range). The examples below show how to read each one.

Reading a P-metric marking: P265/70R17 113T

• P: the tire type. P stands for passenger tire, built to the P-metric standard. A size with no letter in front, such as 265/70R17, is a Euro-metric passenger tire. Its load index works the same way, but its reference pressures are slightly different: 36 psi (250 kPa) for standard load and 42 psi (290 kPa) for extra load.
• 265: the nominal section width in millimetres, measured sidewall to sidewall on a specified measuring rim.
• 70: the aspect ratio. This is not a length. It means the sidewall height is 70% of the section width.
• R: radial construction.
• 17: the rim diameter in inches. Tire sizes mix metric and inch units.
• 113: the load index. This is a code for the maximum load one tire can carry at its reference pressure. Index 113 equals 1,150 kg (2,535 lb).
• T: the speed rating, a code for maximum sustained speed under rated conditions. T equals 190 km/h (118 mph).

Reading an LT-metric marking: LT265/70R17 121/118S Load Range E

• LT: a light-truck tire, built for heavier loads and higher inflation pressures.
• 265/70R17: the same dimensional meaning as on a passenger tire.
• 121/118: two load indexes. The first (121 = 1,450 kg / 3,197 lb) applies when the tire is mounted singly. The second (118 = 1,320 kg / 2,910 lb) applies to each tire in a dual-wheel pair, such as the rear axle of a dually pickup.
• S: the speed rating, 180 km/h (112 mph). LT tires often carry lower speed ratings than passenger tires of the same size.
• Load Range E: the tire's strength and inflation class. It sets the pressure at which the tire reaches its maximum load.

Other load-related sidewall markings

• MAX LOAD / MAX PRESS: the maximum load in kg and lb, plus the maximum permitted inflation pressure. The maximum pressure is a limit the tire must not exceed. It is not the recommended operating pressure.
• SL, XL, Reinforced: on passenger tires, SL means standard load. XL or Reinforced means extra load, which allows more load at a higher reference pressure.
• C suffix (for example 215/65R16C 109/107T): a European commercial tire with single/dual load indexes. It is a separate category from P-metric and LT-metric and uses its own load-inflation tables.

How do load index and load range differ?

The load index is a numeric code for a load in kilograms. The load range is a letter code for the tire's construction strength and the inflation pressure needed to reach its maximum load. Passenger tires use a load index plus SL or XL. Light-truck tires use load indexes plus a load range.

Selected load index values

Load index	Load per tire (kg)	Load per tire (lb)
100	800	1,764
104	900	1,984
108	1,000	2,205
110	1,060	2,337
113	1,150	2,535
116	1,250	2,756
118	1,320	2,910
120	1,400	3,086
121	1,450	3,197
123	1,550	3,417

The pound figure molded on a sidewall can differ slightly from a straight index conversion. This happens because of rounding and the standards organization whose tables the manufacturer follows. When in doubt, the MAX LOAD figure on the tire itself is the reference for that tire.

LT load ranges and their maximum-load pressures

Load range	Ply rating (strength equivalent)	Pressure for maximum load
C	6	50 psi (350 kPa)
D	8	65 psi (450 kPa)
E	10	80 psi (550 kPa)
F	12	95 psi (655 kPa)

The ply rating is a strength equivalent. It does not count the actual number of layers in a modern radial tire.

Why can't you compare P-metric and LT-metric load capacity directly?

P-metric and LT-metric tires use different load-inflation tables and different reference pressures. Because of this, a higher number on one tire does not automatically mean more capacity at the pressure you will run. Three factors cause the mismatch.

1. Different reference pressures

A standard-load P-metric tire's load index describes its capacity at about 35 psi. An LT tire's load index describes its capacity at its load-range pressure, such as 80 psi for Load Range E. If you run an LT tire at 35 psi, it carries much less than its sidewall maximum.

2. Different load-inflation curves

Load-inflation tables show how much load a tire of a given size and type can carry at each pressure. LT tables follow a different curve from P-metric tables. At the same moderate pressure, an LT tire can carry less than the same-size P-metric tire. This is why fitting LT tires to a vehicle designed for passenger tires usually requires a higher pressure than the placard shows.

3. Passenger-tire load derating on light trucks

Passenger-tire load derating is a reduction in a P-metric tire's usable load when it is fitted to a light truck, SUV, or similar vehicle. The tire's rated load is divided by 1.10, so about 91% of the rating remains usable. For example, a P265/70R17 113T is rated at 2,535 lb. On a light truck, its usable load is 2,535 ÷ 1.10 ≈ 2,305 lb per tire.

Side-by-side comparison

Feature	P-metric passenger tire	LT-metric light-truck tire
Example	P265/70R17 113T (SL)	LT265/70R17 121/118S, Load Range E
Load ratings shown	One load index	Single and dual load indexes
Rated load per tire	2,535 lb (1,150 kg) at about 35 psi	3,197 lb (1,450 kg) single at 80 psi
Usable load on a light truck	About 2,305 lb after ÷1.10 derating	No derating; the rating applies as marked
Load strength class	SL or XL	Load range C, D, E, F
Typical operating pressure	Placard pressure, often 30–40 psi	Set from LT load-inflation tables for the actual load
Calculated dimensions	Same formula for the same size numbers	Same formula for the same size numbers

How much load capacity does a replacement tire need?

Each replacement tire must carry at least the load that the vehicle maker's original tire carried at the recommended cold pressure. Each tire must also carry at least its share of the maximum axle load. This information is printed on two labels, usually found on the driver's door jamb:

• Vehicle tire placard: lists the original tire size, load rating, and recommended cold inflation pressure, front and rear.
• Certification label: lists the gross vehicle weight rating (GVWR) and the gross axle weight rating (GAWR) for the front and rear axles.

The axle load per tire is the GAWR divided by the number of tires on that axle. For example, a rear GAWR of 4,000 lb on an axle with two single tires requires at least 2,000 lb of capacity per tire at the pressure you will run.

Steps to compare a passenger and a light-truck replacement

1. Write down the original tire size, its type (P, LT, or Euro-metric), its load index or load range, and the placard cold pressure for each axle.
2. Calculate the minimum load per tire for each axle: GAWR ÷ number of tires on the axle. For dual-wheel axles, use the tire's dual load index.
3. Read each candidate tire's type, load index (single and dual if LT), load range or SL/XL, and MAX PRESS.
4. If the candidate is a P-metric tire and the vehicle is a light truck, SUV, or similar vehicle, divide its rated load by 1.10.
5. Find the candidate tire's load at your intended pressure in the load-inflation table that matches its type. Use the P-metric table for P tires and the LT table for LT tires. Then set a pressure at which the capacity meets or exceeds both the axle requirement and the original tire's capacity, without exceeding the sidewall MAX PRESS.
6. Confirm the tire's speed rating meets the vehicle's requirement. Check the size and fitment separately, as described below.

Replacing LT tires with passenger tires

If a vehicle left the factory on LT tires, replace them with LT tires of at least the same load range. In the comparison above, the P-metric tire's usable load (about 2,305 lb after derating) falls well short of the LT tire's 3,197 lb. This is true even though the size numbers are identical.

Replacing passenger tires with LT tires

LT tires can replace P-metric tires, but only at a pressure that gives at least the original load capacity according to the LT load-inflation table. That pressure is often higher than the placard value. LT tires are usually heavier and stiffer, which can affect ride comfort and handling compared with the original passenger tires.

How do width, aspect ratio, and rim diameter affect tire size?

Sidewall height equals section width × aspect ratio. Overall diameter equals rim diameter plus two sidewall heights. Circumference equals overall diameter × π. These formulas work the same way for P-metric and LT-metric sizes.

Worked example: P265/70R17 or LT265/70R17

• Sidewall height: 265 mm × 0.70 = 185.5 mm. Divide by 25.4 to get 7.30 in.
• Overall diameter: 17 in + (2 × 7.30 in) = 31.61 in (about 803 mm).
• Circumference: 31.61 in × 3.1416 ≈ 99.3 in.
• Revolutions per mile: 63,360 in ÷ 99.3 in ≈ 638.

A P265/70R17 and an LT265/70R17 give the same calculated result. The P or LT prefix changes the load rating and construction, not the size calculation.

Worked example: moving up to LT285/70R17

• Sidewall height: 285 mm × 0.70 = 199.5 mm (7.85 in).
• Overall diameter: 17 in + (2 × 7.85 in) = 32.71 in (about 831 mm).
• Circumference: 32.71 in × 3.1416 ≈ 102.8 in.
• Difference from 265/70R17: about 1.1 in larger in diameter, or about 3.5%.

A 3.5% larger diameter means the tire travels about 3.5% farther per revolution. When the speedometer reads 60 mph, the actual speed would be about 62 mph. This assumes the speedometer was accurate on the original size.

Flotation LT sizes

Some LT tires use inch-based flotation sizes, such as 33x12.50R15LT. The first number is the nominal overall diameter (33 in). The second is the section width (12.50 in). The last is the rim diameter (15 in). No aspect-ratio calculation is needed to estimate the diameter.

What are the limits of calculated tire dimensions?

Calculated dimensions are nominal values from the size code. They are not measurements of a specific tire, and they do not confirm that a tire fits a vehicle.

• Actual dimensions vary. Section width and overall diameter differ between brands and tread designs, and they change with the width of the rim. Manufacturer specification sheets list measured overall diameter and section width on a stated measuring rim.
• Loaded tires are shorter. Under vehicle weight, the distance from the axle to the ground (static loaded radius) is less than half of the unloaded diameter. Tread wear also reduces diameter over time.
• Fitment is a separate check. A tire's approved rim-width range, its clearance to suspension and bodywork through steering and suspension travel, and the wheel's own load and pressure ratings all decide compatibility. Matching geometry does not guarantee fitment, and matching load index does not guarantee adequate capacity.

Key takeaway: Read the size code for geometry and the load markings for capacity, and treat them as two separate questions. A replacement tire passes the load check only when its usable load is at least the per-tire axle requirement and at least the original tire's capacity. Usable load means the rating after any passenger-tire derating, at the pressure you will actually run, according to the correct P-metric or LT load-inflation table.