“What should the tire pressure be when towing?”

This is a common question. If there is one subject among trailer owners that is wrought with misinformation and voodoo, it is tire pressure and load capacity. A discussion about tire pressure is, well, a numbers discussion. Ah, numbers are our friends. Just in case you dislike technical stuff and a bunch of numbers, let me pique your interest with a couple commonly asked but rarely correctly answered questions.

Q. Tire pressures are always shown as cold. Can I set my tires after driving on them and what should I set them at?

A. You should add 5 – 6 psi above the specifications. For example, if the specification is 35 psi, then set your warm tires to 40 or 41 psi.

Q. My SUV/Light Truck came equipped with passenger tires. Is there anything special I should know?

A. Yes. The load carrying capacity of P-Metric tires when used on SUVs, light trucks, and vans is 91% of the stated capacity.

A Real World Case Study

Back in June we replaced the tires on both our travel trailer and our SUV. The SUV came with passenger tires from the factory and were replaced with light truck tires. On the trailer we went from Load Range D tires to Load Range E. This change presents an excellent case study on how to determine what pressures trailer and tow vehicle tires should be set at.

Both the SUV and trailer originally came with radial tires. Non-radial tires, also known as bias ply tires, are difficult to find today. Radial tires comprise the majority of the market and there is no reason not to purchase radial tires. I won’t get into the differences between type and construction methods, pros & cons, etc. After 45 years of working directly and indirectly with the tire industry I can bore you with facts and data, but my preference is based on extensive experience and expertise.

Because the majority of trailers I see are towed by SUV’s and Light Trucks using passenger tires, this will also be a good example to study. Because we upgraded our 2012 Ford Expedition from passenger tires to light truck tires, there will be some good information for everyone who tows a trailer.

What Do Those Numbers Mean?

The original tires on our SUV were size P275/65R18. This is a passenger tire and is commonly referred to as a P-Metric tire. This is what the Owner’s Manual specifies and what the Tire Placard on the door jamb says. A little decoding tells us:

• P = passenger tire
• 275 = the width of the tire in millimeters
• 65 = the aspect ratio – the sidewall is 65% of the tire width
• R = radial tire
• 18 = the diameter of the wheel, in inches, that the tire is installed on

If we look at the tire itself, there is more information than just the size. There is some extra stuff.

M+S P275/65R18 114 T

Using our Secret Decoder Ring…

• M+S = mud and snow tire
• 114 = the max load carrying capacity (2601 lbs.)
• T = maximum speed (118 mph)

Q. Okay, how did I know what “114 T” means?

A. In the US, all specifications for tires and rims are set by the Tire and Rim Association, Inc. (TRA). This is what tire and wheel manufacturers adhere to, and how tires are standardized.

The public cannot access the TRA website, but at the end of this post I’ll provide some links for you to find all this tire information. Load Index Ratings go from 1 – 150. For now, here is the source for the load rating of 114.

Also, in very small print on the side of the tire are the Max Load and Max Air Pressure specifications.

The picture isn’t great, but the specs are:

• Max Load = 2601 lbs.
• Max Pressure = 44 psi

The 91% Rule

Remember that any time a P-Metric tire is installed on a SUV, Light Truck, or Van the Max Load Capacity is reduced to 91% of the capacity.

So for our Ford Expedition, this P-Metric tire has a Max Load of 2,367 lbs. (2601 X 91% = 2367 lbs.).

Vehicle manufacturers are very good at matching tires to their vehicles. If your vehicle came with P-Metric tires, you don’t have to worry about all this math. Set your pressure to specifications and keep all loads (vehicle, hitch weight, trailer weight) within specifications.

But if you change your tire size with another P-Metric tire, you need to go through all these calculations!

Maximum Inflation Pressure

When tire pressure is increased, the load carrying capacity increases to a to a pre-determined point. This point, the maximum load capacity, is what the manufacturer designed it to be. So for our P-Metric P275/65R18 tire the sidewall says Max Load is 2601 lbs. and Max Pressure is 44 psi.

Question: From the 91% Rule above, if I want to ensure the tire can carry 2,367 lbs. capacity, then should I inflate the tire to the Max Pressure of 44 psi per the sidewall of the tire?

Answer: NO! 

With P-Metric tires always follow the Vehicle Manufacturer’s specifications. Our Expedition tire pressure spec is 35 psi. Period.

Wow! So how much weight will each of my tires be rated to carry at 35 psi instead of the Max Pressure on the sidewall of the tire? Well… we have to look at the standards set by the TRA. Here they are below:

Notice that there is no tire pressure beyond 35 psi. Also notice that at 35 psi the max load is 2601 lbs. (or 91% = 2367 lbs. for SUV/LT/Van in the bottom row). So why does the side wall show Max Load = 2601 lbs. and Max Pressure = 44 psi?

Max Load is the maximum the tire is designed to carry, and since the tire meets TRA standards, it will carry 2601 lbs. at 35 psi cold temperature. There is no reason to add more air beyond the 35 psi recommended in our Ford Expedition Owner’s Manual; more air will not add additional load carrying capacity to the tire. Actually the Owner’s Manual states that the Maximum Air Pressure will be higher than Ford’s Recommended Air Pressure.

Many owners of SUVs, Light Trucks, and Vans that are running P-Metric tires are inflating them to the Max Pressure on the sidewall. They are not gaining capacity and do not understand that the recommended pressure on the door placard is the correct weight to handle the maximum capacity of the vehicle. Inflating a P-Metric tire to the Max Pressure may cause handling problems and will cause pre-mature tire wear in the center of the tire.

So why are the numbers (35 vs. 44) different?

The Max Pressure is the maximum pressure the tire is rated to hold. Let’s say you are going to store a vehicle for a couple years, and you want to inflate the tire as much as possible to allow for normal pressure loss over time. Then you would want to inflate the tire to 44 psi.

Sometimes a vehicle manufacturer will recommend a pressure higher than the 35 psi in this example for specific handling characteristics or use. An example might be a special use vehicle that never travels over 10 mph and is only operated on hard paved surfaces… perhaps a meter reader application.

Prove it!

Okay. A while back I wrote a post about trailer tongue weight. In it I shared the actual weights of the SUV’s axles and the Max Allowed by Ford. Here it is below.

The highest weight it at the rear drive axle. Ford specifies that the maximum the rear axle is allowed to carry is 4,250 lbs., or 2,125 lbs. per tire. That is well below the 91% Rule for the P-Metric tire the SUV was originally fitted with at 2,367 lbs.

Bottom line for our SUV using P-Metric passenger tires: the combined weight of the tow vehicle and trailer can be up to 15,000 lbs with the SUV tires set at 35 PSI. In our case we are towing a trailer weighing around 8,500 lbs. and a hitch weight just under 900 lbs.

Light Truck Tires

So why would I want to change from P-Metric Tires to Light Truck Tires? For me, I get more miles from light truck tires versus P-Metric. They have a little more tread when new, and the overall construction is beefier.

Our new tires for the SUV are LT 275/65R18 123 S and are the same size as the original tires. However, they are truck tires. So let’s decode the tire.

• LT = Light Truck use
• 123 = Max Load (Load Range E or 3415 lbs. – there is no 91% rule when using truck tires)
• S = speed rating (up to 112 mph)

What Tire Pressure for Light Truck Tires?

Well, we can skip Ford’s specification on the door because we are no longer running P-Metric tires.

Let’s examine what happened last June when I had the oil changed. I wrote about this last week in the post, but I’ll just copy the poignant point:

“This past June we were camping at Lake Mead. On the way we purchased 4 new trailer tires and 4 new light truck tires. The tire dealer set all 8 tires at 80 PSI. I later double checked the pressures with the tire gauge. These are the maximum load inflation pressures.

“Towards the end of our trip Joyce had to go to work and the camper was left at the campground, driving her home and then returning back to the campground (500 miles round trip). On the way back I stopped at a Ford dealer to get my oil changed.

“When it was time to go home I checked the tires the evening before departure and found that the trailer tires were still at 80 PSI, but the SUV tires were all at 31 PSI. Obviously the Ford technician did not know what he was doing nor understood the relationship of air pressure and temperature — of course few people do…

So what happened?

• The technician checked the door sticker, which said 35 psi
• He did not notice the SUV did not have P-Metric tires
• He reduced the air pressure from 80 psi to 35 psi warm temperature
• When the tires cooled down the pressure decreased to 31 psi cold

How could this have happened? Simple: probably poor training or a lack of management oversight. It’s a management problem.

What could have happened?

I’ll let you decide. But here is the Air Pressure & Load Chart for a LT275/65R18

Since the tires were at 31 psi cold, and the chart does not go below 35 psi, the tires were dangerously under inflated. Probably had a load carrying capacity of around 1800 lbs., and the actual weight on each rear tire was 2,020 lbs. per the chart I posted earlier:

The dealership had created a possible lawsuit or at worse a fatal accident. Fortunately I checked the tires before leaving the campsite, which I always do anyway.

The original weight specification for the P-Metric tire was 2,367 lbs. @ 35 psi cold. Setting these new Load Range E tires at 45 psi will give a max load of 2,310 lbs. which is below Ford’s specs; 50 psi will give us a load carrying capacity of 2530 lbs., which is above Ford’s specification. So 50 psi is the best option. I’ll have to watch for tire wear when not towing the vehicle and may have to adjust a little lower when not towing.

Why did the tire dealer set them at 80 psi? Because I asked them to. I was traveling and did not have the specs with me for the axle weights that were measured on a Cat Scale.

Trailer Tires

Like light truck tires, trailer tires have specs for each 5 psi increment too. However, it is always best to inflate to the Max Load inflation pressure. Let’s take a look at some trailer tire information.

Trailer tires are designed to be positioned on trailer axles only. They are not designed to handle the loads created by drive axles or steering axles. Bias ply tires work well on trailers, but I prefer radial tires on trailers because they have less rolling resistance and run a little cooler. They will run coolest at maximum tire pressure.

If a trailer tire is wearing in the center, the pressure can be reduced using the load table above for your specific sized tire. Keep in mind that most trailer tire failures are due to under inflation.

The combined capacity of all the trailer tires must be equal to or exceed the Gross Vehicle Weight (GVW) specification for the axles of the trailer. I like to have the load capacity of all the tires exceed the total trailer weight by about 20%. Our trailer weighs around 8,600 lbs. fully loaded, so I want the total capacity of the tires to be at least 10,320 lbs. or 2,550 lbs. for each tire. Here is the Tire Inflation & Load Chart for our ST225/75R15 tires. Note that trailer tires use the size designation “ST” versus “P” for passenger tires or “LT” for light truck tires.

Our trailer came equipped with Load Range D tires with a Max Load of 2,540 lbs. that is just below my desired 2,550 lbs., so our replacements are Load Range E with a Max Load of 2,830 lbs.

Trailer tires have a maximum speed rating of 65 mph. If you want a flat tire, drive for extended speeds in hot weather at speeds of 70+ mph. Almost every trailer tire flat I ever had occurred when driving in hot weather at high speeds for an extended period of time. Driving faster than 65 mph will gradually decrease the load carrying capacity of a trailer tire.

Trailer tires are not designed to last a specific amount of miles. The life of a trailer tire is determined by time. Trailer tires should be replaced every 3 or 4 years, regardless of mileage.

Can You Use a Light Truck Tire on a Trailer?

This is a commonly asked question. So let’s do a comparison of ST and LT Load Capacities using the size tire on our trailer, ST225/75R15, which is one of the most common trailer tires on the road. First of all, in this size (LT225/75R15), the highest capacity light truck tire is Load Range D. However, a LT235/70R15 has about the same outer diameter but is slightly wider and can be bought with a Load Range E. So we’ll compare that tire also. See the chart below:

As you can see light truck tires do not have as much load carrying capacity as a similarly sized trailer tire because they are designed to carry more weight using larger cords in the construction of the plies and the belts.