Selecting the Correct Tire Pressure for All Tow Vehicle Classes
|Look for item "A" (Total Front Axle Weight) on your Weight Safety Report.|
|Look for item "B" (Total Rear Axle Weight) on your Weight Safety Report.|
| ||Front Axle|
|Divide item "A" by 2, which is the number of tires on your front axle.|
| ||Rear Axle(s) Options|
|Divide item "B" by 2 if you have a single tire on each end of a rear single axle.|
|Divide item "B" by 4 if you have dual tires on each end of a rear single axle.|
| ||Tire Inflation Weight Range|
|The quotient is the average load weight of each tire load. Use this starting weight range.|
|Find your tire size listed on the correct chart and follow over to the weight range of your tire. (Always use the next weight range above your tow vehicle's tire weight range.)|
|Then follow up the column to select the inflation starting pressure for your tires.|
Item "A," Total Front Axle Weight: 4900 pounds
Number of tires on the front axle: 2
Truck tire size: LT235/80R17
4,900 divided by 2 = 2450 (round up any fraction)
On the LT235/80R17 tire chart the next weight range is 2545 (Read the "SINGLE" line.)
The starting tire inflation pressure is 60 PSI.
Item "B," Total Rear Axle Weight: 7000 pounds
Number of tires on the rear axle: 4 (dually axle)
Truck tire size: LT235/80R17
7,000 divided by 4 = 1750 (round up any fraction)
On the LT235/80R17 tire inflation chart the next weight range is 1870 (Read the "DUAL" line.)
The starting tire inflation pressure is 45 PSI.
TRUCK TIRE INFLATION TABLES
UPDATED! Selecting the Correct Tire Pressure for Your Trailer
We at Fifth Wheel St. no longer recommend adjusting trailer tire inflation pressure below the allowed maximum PSI based on actual loaded scale weight on individual tire or axle positions.
However, we do strongly recommend weighing individual trailer tire positions to ensure none of the axles or tire positions are overloaded. Reports have shown that trailers do not have equal weight across all tire positions. Some RV load configurations may reveal as much as 20% difference between the front and rear axle. This especially true for Toy Haulers. It is possible that mismanaged trailer load distribution will cause one end of an axle or a tire to be overloaded. It has been stated, but never confirmed by any RV Weighmaster, that there are many RVs traveling on the road with at least one tire or axle side overloaded. The only way to ensure tires and or axles are not overloaded is to weigh each tire position on your trailer. Unfortunately, attempting to obtain accurate individual tire position weight is practically impossible at all truck scales. View our list of recommend RV Weighmasters here.
Our recommendation is this: All trailer tires should be inflated to the allowed maximum PSI as follows:
|First priority: The posted trailer certification label in accordance to the minimum tire requirement stated on the Certification Label. View the example on the right.|
|Second priority: The tire's maximum PSI molded on the tire's sidewall. View the example on the right.|
Never exceed the maximum inflation rating molded on the tire's sidewall.
Never install tires rated less than required per the Certification Label.
Never exceed the maximum inflation rating for the wheels/rims or valves when setting the tire cold inflation pressure.
Read about the differences in Special Trailer (ST) tires vs. Light Truck (LT) and Passenger (P) tires below.
The answer to "Why Max Inflation?"
Bottom line... FIRST
When a radial tire is loaded, the belts and body have to bend from a round shape to a flat shape in the area that contacts the road. Additionally, when you turn a corner, the forces generated to move the RV sideways have to be transferred through the tire structure. This causes additional bending of the belt and body structure. Increased bending causes increased stretching of the rubber. With enough stretch, microscopic cracks form and existing cracks get bigger. Eventually with enough cycles and enough force, cracks may grow and this increases the possibility of tire components separating which could lead to a tire failure. You can lower tire stretching if you decrease bending and you can decrease bending if you increase tire inflation.
Engineer Speak and Techno Babble
If you own a multi-axle trailer, tire bending forces can be much higher than those seen on a tow vehicle, motorhome or car. Tires on these types of vehicles are not close together and are located at the corners of the vehicle.
Special consideration for multi-axle trailers. Warning, this gets technical.
When not driving in a straight line, there are significant side loads on multi-axle trailers because the tires are fighting each other; this is because the axles are not "pointed" to the center of the radius of the turn. These loads cause interior structural tearing. Sometimes, loads may be 20% or more higher than those seen in tires on non-trailer applications. Initially, tearing begins at the microscopic level, but with time and repeated cycles, these microscopic tears grow, which can lead to small cracks at the belt edges as seen here at the arrows.
If not spotted, these cracks continue to grow to almost the full width of the tread as seen below.
If you are lucky, you will see the bulge in the tread as seen on the left. Clearly, you know this tire has failed and MUST be removed AT ONCE as the separation can grow and can cause a belt to come off the body of a tire.
You can lower these forces by either decreasing the load by at least 20% on the tire (probably not something you want to do or may not be able to do) or you can increase the inflation to stiffen the structure which will decrease the slip-angle. In this case, you could increase the tire inflation from the minimum inflation needed for the static load to the inflation associated with the max tire load as molded on the tire sidewall. BUT, you need to be sure you are not exceeding the max rating of the wheel/rim.
Therefore, the best recommendation I can give to trailer owners is to run the max inflation molded on the tire sidewall. For owners of a tow vehicle or motorhome, I recommend you run the inflation needed to carry the actual measured tire load plus at least a 10% margin.
About Roger Marble:
Before retiring, Roger spent 40 years in the tire industry working for a major manufacturer developing tires for applications in North, Central and South America. During his career he worked on many kinds of tires - heavy truck, passenger, light truck and Indy-car types. If you have a question for Roger, email him at tireman9(at)gmail.com. Also, visit his blog for additional RV tire safety information at RV Tire Safety.
Did you know...
That visual tire failure indicators may not be seen until months after the initial cause?
Before blaming the tire manufacturer after a blowout, consider the time you discovered the tire had been underinflated after your travels on the road, or some other event that may have triggered the failure process.
The ST Difference
The construction, design, materials and testing used in ST Special Trailer tires meet the higher load requirements, duty cycles and special demands of trailering.
Polyester cords in an ST tire are bigger than in a comparable P or LT tire.
Steel cords used in ST tires have a larger diameter and greater tensile strength to meet additional load requirements.
ST tire rubber compounds contain chemicals to resist weather and ozone cracking, particularly conditions resulting from extended storage and the unusual duty cycles of trailer tires.
The slightly shallower tread depth of a trailer tire reduces sway and rides cooler, which adds to tire longevity.
ST tires feature stiffer sidewalls, especially in the lower section which:
Reduces sidewall flexing causing the trailer to track straighter.
Diminishes the risk of trailer sway.
Lessens the risk of sidewall puncture and blowout.
ST tires generally offer approximately 10% percent more load capacity than a similar LT tire and nearly 40% more than a P passenger tire.
This information is published in Trailer Tires: Tips & Best Practices.
Please note, all special trailer (ST) tires are rated for the maximum speed of 65 miles per hour.
RV expert, Mark Polk, wrote an excellent article about using LT tires on trailers. Check it out here.