Tires for Towing: Load, Pressure and Trailer Tires

Towing puts more load on tires than ordinary driving, on the tow vehicle and on the trailer. This guide covers the tire side of towing: the ratings on the sidewall, the pressures to use, how to check them, and why trailer tires are a separate topic.

It is not a towing course, and it does not tell you how much you can tow. That limit is in your owner's manual and on your vehicle's labels. When this guide and your manual differ, follow the manual.

Two sets of tires, two sets of numbers

When you tow, there are two groups of tires to think about.

• The tow vehicle's tires carry the vehicle, its passengers and cargo, and part of the trailer's weight. USTMA's manual explains that the weight a trailer puts down on the hitch (tongue weight) is supported by the towing vehicle's tires, not the trailer's.
• The trailer's tires carry the rest of the trailer and its load.

Each set has its own size, load rating and pressure. Federal rules require a trailer to carry its own tire information, separate from the tow vehicle's placard (49 CFR 571.110). Use the right label for the right tires.

Tow vehicle: start with the placard

Federal rules require a tire placard on the vehicle. It shows the maker's recommended cold inflation pressure and the size of the tires it came with. On many vehicles it is on the driver's side door pillar. The owner's manual lists the same information.

General Motors' owner's manual says tires must be properly inflated to support loads while towing a trailer. It also says the recommended pressure on its label is the minimum needed to support the vehicle's maximum load carrying capacity.

Standard and loaded pressures

Michelin says the placard typically lists both a standard pressure and a loaded pressure, and that pressures can differ front to rear. For a full passenger load, heavy cargo or towing, it advises the higher loaded recommendation. For ordinary driving, go back to the standard one.

If your placard or owner's manual gives only one figure, use it. If it gives a separate figure for towing or for a full load, use that. Do not guess a higher number.

The sidewall number is a ceiling

For the tow vehicle, Continental and Michelin both say the maximum pressure on the sidewall is not the target. It is the most the tire can take. USTMA says the same. Do not inflate the vehicle's tires to the sidewall maximum just because you are towing.

Never set pressure below what your placard or manual gives, either. USTMA warns that underinflation causes heat build-up that can lead to tire failure.

Load index: what it does and does not do

The load index is the number just after the size, such as the 97 in P225/60R16 97T. USTMA calls it a numerical code associated with the maximum load a tire can carry. Our guide to load index and speed rating explains how to read it.

Three points matter when you tow.

• Match or exceed the placard. USTMA says to replace a tire with a load index equal to or above the index required on the placard.
• Pressure matters. A tire's capacity depends on its inflation pressure. Toyo says LT tires, which have higher ratings, offer no benefit as "heavy duty" tires when underinflated.
• A higher number does not raise your vehicle's limits. USTMA says an axle's rating is set by the lowest design capacity of any component (tires, wheels, brakes, springs or axle). If the wheels are the lowest, installing tires with higher load capacity does not increase it. Federal rules require the tires on each axle to be rated for at least that axle's gross axle weight rating, but the rating itself comes from the maker.

If your tow vehicle uses LT tires, our guide to LT vs P-metric tires covers load range and why LT and P-metric tires are not interchangeable.

Speed rating: a test result, not advice

The speed rating is the letter after the load index. USTMA says it relates to laboratory tests and is not applicable if tires are underinflated, overloaded, worn out or damaged. USTMA does not endorse operating any vehicle in an unsafe or unlawful manner, and says speed ratings do not imply a vehicle can be safely driven at the tire's maximum speed.

For towing, two things follow.

• Posted limits and your manuals come first. Check your owner's manual and the trailer's manual for any speed guidance.
• The trailer's tires may have a lower rating than the truck's. General Motors' manual says to always know the maximum speed rating for the trailer tires before driving, because it may be significantly lower than the vehicle tire speed rating. If the sidewall does not show one, GM's manual gives a default of 65 mph for trailer tires.

Trailer tires are different

Tires made for trailers have ST on the sidewall, short for special trailer. USTMA says tires specifically designed for trailer use in highway service carry ST, and that ST tires have different load capacity characteristics. It adds that ST tires should not be replaced with any other type of tire. Federal rules allow trailers to be equipped with ST tires.

General Motors' manual describes trailer tires as having stiff sidewalls to help prevent sway and support heavy loads. It says those features can make it hard to tell by eye whether a trailer tire is low.

Do not mix them up

• Do not use the tow vehicle's pressure on the trailer. The trailer has its own tire information. General Motors' manual points to the trailer tire placard on the trailer or the trailer tire sidewall.
• Do not swap tire types between trailer and truck. Your tow vehicle's placard names the tire size for the truck, so follow it. None of the sources we read endorse fitting ST tires to a tow vehicle.
• Read the sidewall. The load rating is on the trailer tire's sidewall, and General Motors' manual says never to load a trailer with more weight than its tires are designed to support.

Trailer tires age

General Motors' manual says trailer tires deteriorate over time and that many trailer tire makers recommend replacing tires more than six years old. The sidewall shows the week and year of manufacture. Our guide to the tire date code shows how to read it, and when to replace tires covers wear.

A valve detail

USTMA warns that a valve rated for 65 psi should not be used on a tire rated for more, including ST tires. A high-pressure valve is needed above 65 psi. If your trailer tires run at higher pressure, ask a tire professional to confirm the valves.

Check pressure cold, every trip

USTMA's advice, repeated by Transport Canada, is to check when tires are cold. USTMA defines that as the vehicle parked for at least three hours or driven less than one mile at moderate speed. Transport Canada gives three hours stationary, or driving no more than 2 km.

• Check before each trip. USTMA says to check before long trips, before carrying heavy loads and before towing a trailer, even on a vehicle with a tire pressure monitoring system. Transport Canada says to always measure before a long trip. General Motors' manual says to check all trailer tire pressures before each trip when the tires are cool.
• Check the trailer too, and the spare. USTMA and Transport Canada both include the spare.
• Never let air out of hot tires. USTMA says it is normal for pressure to rise above the cold figure while driving. A hot reading at or below the cold figure may mean the tire is dangerously low.
• Use a gauge you trust, with a range that suits the pressure you run. USTMA mentions a dual head gauge reading to 100 psi for light truck tires. See our guide to digital and dial gauges, and our tire pressure and TPMS basics.

Transport Canada calls underinflation the number one enemy of a tire. It also warns that running an underinflated or overloaded tire at highway speeds on a warm day is a recipe for tire failure.

The spare

Vehicle makers differ on spares for towing. General Motors' manual says not to tow a trailer while using a compact spare. Ford's towing guide says a conventional, identical full-size spare is required for trailer towing. Check your own manual.

Some vehicles can monitor trailer tire pressure. Ford's guide lists an optional trailer tire pressure monitoring system, and General Motors' manual describes one that needs compatible sensors. Whether yours can is in your manual. It is a supplement to checking with a gauge.

Weight ratings: where to look

Tires are one part of the weight picture. These are the terms vehicle makers use. Ford's towing guide defines them, and General Motors' manual covers the same ground.

• Payload is the maximum weight of cargo and passengers the vehicle is designed to carry. Ford says trailer tongue load counts as cargo.
• GVWR (gross vehicle weight rating) is the maximum allowable weight of the fully loaded vehicle. General Motors' manual says tongue weight is part of the weight the vehicle is carrying.
• GAWR (gross axle weight rating) is the most a single axle should carry.
• GCWR (gross combined weight rating) is the maximum allowable weight of the loaded tow vehicle and loaded trailer together.

The numbers for your vehicle are in the owner's manual and on its certification label and trailering information label. Ford and General Motors both say the actual weights must stay within every one of these ratings, not only the maximum trailer weight.

General Motors' manual says the only way to be sure is to weigh the tow vehicle and trailer fully loaded for the trip. USTMA says the same of RVs, and notes you can be within the overall rating but still overload one axle end or one tire. Public scales at truck stops and similar sites are one option, and USTMA suggests allowing time.

Before you go: a short checklist

• Tow vehicle cold pressure set from the placard or manual, using the loaded figure if one is given.
• Trailer cold pressure set from the trailer's label and tire sidewall.
• Spare checked, and your manual consulted on spare use.
• Trailer tire date codes checked.
• Loaded weights within your vehicle's GVWR, GAWR and GCWR, confirmed by weighing if you are near a limit.
• Tow vehicle tires match the placard in size, load index and speed rating.

If a tire is damaged, worn, old or repeatedly losing air, have a tire professional look before you tow. To find tires for your tow vehicle, start with your vehicle or the tire finder. If you are changing tire size, check the effect with the tire size calculator and the speedometer calculator, then confirm load with the placard. For tread choices on a truck that also works off-road, see all-terrain vs highway tires.

Where to look

TireShopper does not test products. These links go to Amazon search results, not to a particular product. Check the gauge's pressure range and your inflator's maximum pressure against the figures on your placard and trailer label before you buy:

• Tire pressure gauges that read to 100 psi (Amazon search, opens in a new tab)
• Portable tire inflators, 12V plug-in (Amazon search, opens in a new tab)

Our guide to choosing a portable tire inflator explains maximum pressure and duty cycle.