LT vs P-Metric Tires: What the Letters and Load Ranges Mean

Pickups and SUVs can wear three kinds of tire: P-metric, Euro-metric or LT. The sizes can look almost the same. The load ratings and the air pressures behind them are not.

This guide explains what the letters mean, how load range works, why LT tires run at different pressures, and how to read a size such as LT275/65R18 123/120S. The rule that runs through all of it is to follow your vehicle's tire placard and owner's manual.

What the letters mean

The letters before the width tell you which sizing system a tire follows. USTMA, the U.S. tire manufacturers' trade group, lists them in its tire care manual.

• P is P-metric, meaning passenger.
• LT is light truck.
• ST is special tire for trailers.
• No letter is metric, also called Euro-metric.

Yokohama puts it this way. P-metric sizes start with P and follow Tire and Rim Association standards. Sizes with no letter follow the European standard. Euro-metric sizes are dimensionally equivalent to P-metric sizes, but their load-carrying capabilities typically differ slightly. LT marks a light truck metric size, for vehicles that carry and tow more than passenger cars.

Federal rules define an LT tire as one its maker designates as primarily intended for lightweight trucks or multipurpose passenger vehicles (49 CFR 571.110).

A P-metric tire on a truck is common

USTMA notes that P-metric and Euro-metric sizes are often fitted to pickups, vans and SUVs. A vehicle that looks like a "light truck" may not use LT tires at all. The placard tells you which type your vehicle was built for.

Why similar sizes are not equal

Toyo shows how far apart the systems can sit. A P225/55R17 95T is rated for 1,521 pounds at 35 psi. A 225/55R17 97T, a Euro-metric size, is rated for 1,609 pounds at 36 psi. Toyo adds that tires with the same load index may carry the same load, but not always, and may need substantially different pressures.

The same reasoning applies to 265/70R17 and LT265/70R17. Same digits, different tires.

Load range: C, D and E

LT tires carry a load range letter. USTMA says the load range identifies the tire's load and inflation limits. It also says light truck tires for light vehicles typically come in load ranges B, C, D and E. Yokohama adds that most tires on the market are load range C, D or E.

Toyo's catalogs label C as a 6-ply rating, D as 8-ply and E as 10-ply. Think of that as a rating label. The tire's load and pressure limits are what matter, and they are printed on the sidewall.

Toyo's reprint of the Tire and Rim Association's light truck table shows the pattern for a size such as LT265/65R17. Load range C tires reach their maximum load at 50 psi, load range D at 65 psi and load range E at 80 psi. USTMA's own examples show a load range E tire with a maximum of 80 psi.

So a higher letter generally means a higher maximum load and a higher maximum pressure. It does not mean you inflate to that maximum.

The sidewall number is a ceiling

USTMA says the pressure on the sidewall is the maximum allowed in the tire, whatever the vehicle or load, and should not be used on its own to set pressure for a specific vehicle. Michelin says the same about its MAX LOAD and MAX PRESS markings. Use the cold pressure on your placard or in your owner's manual.

One practical note on valves

USTMA warns against using a valve rated for 65 psi on an LT or ST tire rated for more than that. A high-pressure valve is needed above 65 psi. When LT tires are installed, ask the shop to confirm the valve matches the tire's maximum pressure.

How to read LT275/65R18 123/120S

Take the size one piece at a time. LT275/65R18 breaks down like this.

• LT means light truck metric.
• 275 is the section width in millimeters.
• 65 is the aspect ratio, so the sidewall height is 65 percent of the width.
• R means radial.
• 18 is the wheel diameter in inches.

After the size comes the service description, 123/120S.

• 123/120 are two load indexes. Yokohama says the first is the maximum load for a single wheel and the second is for dual-vehicle applications, where two tires share an axle end. USTMA says the same.
• S is the speed symbol. In USTMA's table, S corresponds to 112 mph.

For a sense of scale, USTMA's bulletin on 17-inch LT sizes lists the same 123/120 pair for LT265/70R17 load range E. At the rated 80 psi, that is 1,550 kg (3,415 lb) in single use and 1,400 kg (3,085 lb) in dual use. Those are limits for that tire at its maximum pressure, not figures for your truck.

Our guides to load index and speed rating and reading a tire sidewall cover the rest of the markings.

Why LT tires run at different pressures

A tire carries its load on air. USTMA says light truck tires usually operate at higher inflation pressures and carry greater loads on a regular basis, and that they are typically designed for more severe service. Toyo says the load a tire supports depends on its inflation pressure and its maximum load and inflation capability.

Toyo's bulletin gives a worked comparison. A P245/75R16 109 carries 2,271 pounds at 35 psi. To carry that same load, an LT245/75R16 in load range C must be inflated to 50 psi. Toyo says load range D or E needs the same 50 psi for that load. It also warns that LT tires offer no benefit from being "heavy duty" when underinflated.

That example is why you should not carry a pressure from one type of tire to another. It also shows why the placard matters. It states the maker's recommended cold pressure for the tires that came on the vehicle.

If the tire type or size changes

If you move to a different size or type, the pressure that carries the original load may change. Toyo's guidance is to read the load at the placard pressure for the original tire in the Tire and Rim Association tables, then find the pressure at which the replacement carries at least that load. Tire makers publish these tables, and a shop can work it out for you.

Never go below the vehicle maker's recommended pressure, and never exceed the sidewall maximum. If the pressure changes, ask the shop to tell you the new number, and note it somewhere you will see it.

For checking and setting pressure at home, see our guide to tire pressure and TPMS basics.

When does a truck or SUV need LT tires?

There is no rule of thumb based on the size of the vehicle. Your vehicle maker decides, and the placard shows it.

• If the placard lists an LT size, replace with an LT size that carries equal or more load at the placard pressure. BFGoodrich says never to replace original LT tires with P-metric tires. Doing so loses load capacity and durability. USTMA says it is not recommended unless the vehicle maker specifies otherwise.
• If the placard lists a P-metric or Euro-metric size, use that type unless you have checked an LT option with the vehicle maker or a tire professional.
• If your owner's manual calls for a specific tire, that wins over anything on this page.

Michelin and BFGoodrich both describe LT tires as built for towing and heavy payloads, with a stiffer construction and higher load capacity. That is a reason to ask about LT tires if you tow or haul. It is not a reason to change type without checking.

The 1.10 reduction for P-metric tires on trucks

Federal rules reduce the load rating of a passenger-type tire used on a light truck. Toyo's bulletin says the reduction is a factor of 1.1, applied at any inflation pressure. For example, a 305/50R20 with a maximum load of 3,086 pounds is treated as 2,805 pounds on a light truck. The factor is not needed for LT-designated tires.

Federal rules (49 CFR 571.110) describe the same division by 1.10 when passenger tires are fitted to a truck or SUV. If you replace the original tires with the same type and size, the vehicle maker has already accounted for it.

Going from P-metric to LT

USTMA says that in some cases original P-metric tires can be replaced with LT tires, but it points to several checks first. Ask the vehicle maker whether the wheel suits the higher pressures. Check rim width, body and chassis clearance, and note that handling may be affected. A significant pressure adjustment is needed to carry the same load as the original size. New wheels may be required.

Do not mix types

USTMA's tire care manual says never to mix P-metric or Euro-metric passenger tires with light truck tires on the same vehicle. It recommends all four tires match in size, load index, speed rating and construction, and notes four-wheel and all-wheel drive vehicles need all four tires the same size unless the owner's manual says otherwise.

Ride and fuel economy: what makers say

Tire makers describe the trade-off in general terms, without numbers.

• Toyo says the higher pressure LT tires need can make the ride harsher, which is why LT tires may not suit a vehicle that came with P-metric tires.
• USTMA says light truck tires tend to be heavier than passenger tires.
• BFGoodrich says new SUVs and light trucks often come with P-metric tires to meet fuel economy goals, and that LT tires are chosen for towing, hauling and off-road use.

Ride, weight and efficiency vary a lot between individual models. Compare specific tires rather than the type. If you mostly commute and rarely carry or tow much, the type on your placard is built for that. If you carry or tow near your limits, your manual and placard show the load your tires must support.

A short checklist

• Which size and type is on my placard: P-metric, Euro-metric or LT?
• What cold pressure does the placard give for each axle?
• Does the replacement have a load index, and load range if it is LT, at least equal to the original at the placard pressure?
• Am I changing type, size or load range? If so, who confirmed the pressure?
• Am I mixing types? Do not.

Try the tire size calculator to compare diameters when you consider a different size. It does not check load, so confirm that with the placard and a tire professional. If you tow, read our guide to tires for towing next. For off-road tread choices, see all-terrain vs highway tires. To start from your own vehicle, use the tire finder or browse vehicles.