Tire load index chart and axle check

What the load index is

“A tyre’s load index is a number that can be used to look up the maximum amount of weight that a tyre can bear at its speed rating when inflated to its maximum pressure.” [Pirelli kg chart]

On the sidewall it comes right after the size, together with the speed symbol: in P225/65R17 102H, 102 is the load index and H the speed rating. The index itself is not a weight; the chart below turns it into one. Pirelli's own example:

“A tyre with a load index of 91, for example, can hold a maximum load of 615 kg at the correct PSI up to its rated speed.” [Pirelli kg chart]

That is 1,356 pounds. The service description is not on every tire, but the maximum load is. Bridgestone notes:

“This is because the load index is not required by law to be printed on tires.” [Bridgestone tire size]

The federal standard for passenger and light-truck radial tires does require the sidewall to show:

“The maximum load rating and for LT tires, the letter designating the tire load range;” [49 CFR 571.139]

So when the index is missing or hard to read, the maximum load on the sidewall answers the same question.

Load index chart, pounds and kilograms

Kilograms are Pirelli's figures [Pirelli kg chart]. Pounds are worked out from them: one pound is exactly 0.45359237 kg, so pounds = kg ÷ 0.45359237, rounded to the nearest pound. Pirelli's own pound chart [Pirelli lb chart] agrees with that arithmetic to within a pound on every row except 131, 135, 148, where its two charts disagree; those rows are left blank here. For them, read the maximum load molded on the sidewall.

A load index rating holds only at the right pressure:

“A tire’s load index rating is only valid if the tire is inflated to its maximum cold inflation pressure.” [Continental load index]

Will two tires carry the axle? The federal rule

Federal Standard No. 110 sets the tire load rules for lighter vehicles:

“This standard applies to motor vehicles with a gross vehicle weight rating (GVWR) of 4,536 kilograms (10,000 pounds) or less, except for motorcycles, and to non-pneumatic spare tire assemblies for those vehicles.” [49 CFR 571.110]

For multipurpose passenger vehicles (MPVs), trucks, buses and trailers it compares the tires on each axle with that axle's rating:

“Except as provided in S4.2.2.2, the sum of the maximum load ratings of the tires fitted to an axle shall not be less than the GAWR of the axle system as specified on the vehicle's certification label required by 49 CFR part 567.” [49 CFR 571.110]

Passenger car tires count for less on those vehicles:

“When passenger car tires are installed on an MPV, truck, bus, or trailer, each tire's load rating is reduced by dividing it by 1.10 before determining, under S4.2.2.1, the sum of the maximum load ratings of the tires fitted to an axle.” [49 CFR 571.110]

The standard defines a passenger car tire this way (a P-metric or metric size with no letter in front; LT tires are not reduced):

“Passenger car tire means a tire intended for use on passenger cars, multipurpose passenger vehicles, and trucks, that have a gross vehicle weight rating (GVWR) of 10,000 pounds or less.” [49 CFR 571.110]

On a passenger car, the rule is per tire:

“The vehicle maximum load on the tire shall not be greater than the applicable maximum load rating as marked on the sidewall of the tire.” [49 CFR 571.110]

The vehicle's maximum load on each tire is not printed anywhere you can read. The axle rating is, so the checker above uses half of it per tire: a tire that covers half the axle's rating also covers the car's own share of that axle. The axle rating is on the certification label, which federal rules place here:

“the label shall be affixed to either the hinge pillar, door-latch post, or the door edge that meets the door-latch post, next to the driver's seating position” [49 CFR 567.4]

““Gross Axle Weight Rating” or “GAWR,” followed by the appropriate value in pounds, for each axle, identified in order from front to rear (e.g., front, first intermediate, second intermediate, rear).” [49 CFR 567.4]

The same label says which kind of vehicle it is, which decides whether the 1.10 reduction applies:

“The type classification of the vehicle as defined in § 571.3 of this chapter (e.g., truck, MPV, bus, trailer).” [49 CFR 567.4]

Worked example

Example figures, not a particular vehicle: a used P225/65R17 102H for the front axle of an SUV whose label classifies it as an MPV and gives a front GAWR of 2,900 lb.

1. Load index 102 = 850 kg = 1,874 lb per tire.
2. It is a passenger tire on an MPV, so it counts for 1,874 ÷ 1.10 = 1,703.6 lb.
3. Two tires: 2 × 1,703.6 = 3,407 lb, against a GAWR of 2,900 lb: 507 lb to spare.

On a passenger car with the same axle rating the tire would count in full: 2 × 1,874 = 3,748 lb.

Never lower than the original

Tire makers put the replacement rule plainly:

“Never fit tires with a lower load index than the factory specification.” [Michelin load and speed]

“You should never install tires with a lower load index than what the vehicle manufacturer specifies.” [Continental load index]

“Your new tire's load index must meet or exceed the original specification.” [BFGoodrich]

One caution when you compare a passenger tire with an LT tire, or a standard-load tire with an extra-load one:

“The load index should not be used independently to determine replacement tire acceptability for load capacity.” [Mastercraft]

Tires of different types reach their rated load at different pressures, so for a change of type ask the shop for the tire maker's load and inflation table.

Light truck tires: two numbers and a load range

An LT tire often shows two load indexes, such as 120/116:

“The first number is the load rating when the tire is used alone, and the second is the rating when it is used as a dual tire.” [Pirelli lb chart]

It also carries a load range letter:

“The load range identifies the tire's load and inflation limits.” [Mastercraft]

Falken explains the letters in terms of the older ply ratings and pressures:

“So typically we would say a tire is 6-ply rated, 8-ply rated, or 10-ply rated, and those correspond to Load Range C, D, and E respectively. So of course a Load Range F does correspond to a 12-ply rating.” [Falken]

“So typically that Load Range C, for example, carries a range of loads at 35 to 50 PSI. Load Range D, up to 65 PSI. Load Range E, up to 80 PSI.” [Falken]

“The 12-ply rating doesn't actually mean that the tire has 12 different plies or 12 layers.” [Falken]

Dual-wheel axles

Standard No. 110 covers dual-wheel axles too: it adds up the load ratings of all the tires fitted to the axle, which on dual wheels is four tires. Each of them is mounted as a dual tire, so each counts at its dual rating, the second number (the 116 in 120/116). Set “Tires on this axle” to four in the checker and it uses the dual index, or the dual maximum load you type from the sidewall. For one tire on each side it uses the first (single) index. Vehicles with a gross vehicle weight rating over 10,000 pounds are outside Standard No. 110: ask a tire professional.