RV Tires: Age, Load and Pressure

Tires are among the parts of an RV most likely to fail without warning, and on an RV the cause is often age rather than tread depth. A tire that looks fine can be years past the point where it should be replaced. The DOT code on the sidewall tells you when it was made; the load and inflation tables tell you how much air it needs for the weight it actually carries. Check both before any trip, and check them again after storage.

Read the DOT date code first

Every tire sold in the US carries a DOT code on the sidewall. The last four digits give the week and year of manufacture: 2419 means the 24th week of 2019. That is the only reliable way to know a tire’s age. The date on the title or the seller’s word does not count.

RV tires often age out before the tread wears out. Tire makers’ advice varies, but common guidance is a yearly inspection once tires are 5 years old and replacement no later than 10 years from the date of manufacture, whatever the tread looks like. On a used RV, check every tire, including the spare. A spare that has been sitting in the sun for years is not a usable spare.

Why age matters more than tread

Rubber compounds harden and crack with exposure to sunlight, ozone and heat, even when the RV is parked. A tire with deep tread but a sidewall full of small cracks is not safe at highway speed. UV light and long storage are the usual causes. Covers, shade and regular use help, but they do not reset the clock.

ST vs LT tires: what the letters mean

Trailer axles use ST (special trailer) tires. ST tires are built for trailer service and are not meant for drive or steer axles on a motor vehicle. ST tires without a speed rating are limited to 65 mph. That limit is a real constraint on a long trip, and it is one reason trailer tires are often the weak point in a towing setup.

Light truck (LT) tires are built for trucks and vans that carry loads, and are fitted to many tow vehicles and van- or truck-based motorhomes; larger motorhomes often use heavier commercial truck tires. Motorhomes use tires sized for the chassis and the load, and the vehicle’s tire placard gives the correct inflation for the original equipment. Do not substitute a passenger-car tire for either an ST or an LT tire. The load range and the sidewall construction are part of the rating.

Load, inflation and the only number that counts

The pressure molded on the sidewall is the maximum for the tire at its rated load, not the pressure for your RV. The correct pressure comes from the actual load on each axle, read from the tire maker’s load and inflation table or the vehicle’s tire placard, and measured cold.

To know the actual load, weigh the RV fully loaded, by axle where possible, at a public scale. Guessing from the unloaded vehicle weight (UVW) is not enough: water, propane, gear and passengers all add up. Water weighs about 8.3 pounds per gallon and propane about 4.2 pounds per gallon, so a full fresh tank and full cylinders are part of the load. The cargo carrying capacity (CCC) on the weight label is the budget you are working within.

Underinflation is the common failure. A tire that is low on air flexes more, heats up and can fail. Overinflation raises the pressure beyond what the load needs and gives a harsher ride with less contact. Check pressures cold, before driving, and adjust for the load you are actually carrying.

Weighing a towable and a motorhome

For a travel trailer or fifth wheel, the loaded trailer weight and the tongue or pin weight matter. Tongue weight of a conventional travel trailer is commonly 10 to 15 percent of the loaded trailer weight; fifth-wheel pin weight is commonly 15 to 25 percent. Those figures belong in the towing plan, not just the tire plan. The towing a trailer guide covers hitches, brakes and sway control alongside the weight ratings.

For a motorhome, weigh each axle and compare against the gross axle weight rating (GAWR). If an axle is over its GAWR, move or remove cargo; more air pressure does not raise an axle’s rating.

Sun, storage and the damage you cannot see

Tires age fastest when the RV sits still in the sun. Flat spots from long storage and cracked sidewalls are the usual damage. Covers, shade, correct pressure and regular checks prevent most of it. Move the RV periodically if it is stored for long periods, and keep tires out of direct sun where possible.

A yearly inspection once tires reach 5 years old is the common guidance. Look at both sidewalls, the tread and the valve stems. Cracks, bulges, cuts and exposed cords are reasons to replace, not repair. A tire that has lost pressure and been run flat may have internal damage that is not visible.

When a tire must be replaced now

Replace a tire immediately if any of these are present:

• Cracks or weather checking in the sidewall, especially deep cracks.
• A bulge or bubble anywhere on the sidewall or tread.
• A cut, puncture or impact damage that reaches the cords.
• Tread depth at or near the wear indicators.
• A DOT date code showing the tire is at or beyond the replacement age guidance.
• A valve stem that is cracked, leaking or loose.

A tire that has been run while flat should be inspected by a tire professional before it goes back into service. Do not rely on a visual check alone after a pressure loss.

What goes wrong: common mistakes and red flags

• Buying on tread depth alone. A used RV with deep tread and old tires is a breakdown waiting to happen. Check the DOT code on every tire before purchase.
• Using the sidewall maximum as the running pressure. The correct pressure is for the actual load, measured cold. The sidewall number is a limit, not a target.
• Ignoring the trailer speed limit. ST tires without a speed rating are limited to 65 mph. A tow vehicle that cruises faster is overdriving the trailer tires.
• Mixing tire types on an axle. Keep matched tires across an axle, and do not put a passenger tire on a trailer axle or a motorhome.
• Skipping the spare. A spare that is old or underinflated is not a spare. Check it with the rest.
• Storing in the sun without covers. On an RV that sits, UV and heat can age tires faster than mileage does.
• Not weighing the RV. Without a scale reading, the pressure and the load margin are guesses.

A used RV inspection should include the tires, the DOT codes and the spare. The used RV inspection checklist covers the rest of the walk-around, and the RV maintenance schedule sets out the seasonal checks that keep tires and the rest of the RV in service.

Tire replacement, wheel work and any repair to the tire or wheel assembly is a job for a qualified tire professional. Work on 120-volt or 240-volt wiring, the converter, inverter or generator is a job for a qualified RV technician, and propane system work is likewise for a qualified RV technician.