Safe Tire Pressure

Safe Tire Pressure

Now that we are traveling more, I would like to spend less time worrying about tires and more time focused on fun.   We have had some notable tire failures we don’t want to repeat.

Tire Failure on Fifth-Wheel
Tire Failure on Fifth-Wheel

From the many flats and worse (see above) we had on our fifth-wheel, we learned a few important lessons the hard way:

  • Keep the tires inflated at the right pressure
  • Roll the tires at least every two weeks by driving the rig around
  • Buy tires with the highest load range possible
  • At every stop, walk around the truck and trailer feeling every tire for excessive heat

After switching to a motorhome, I learned how to put air into dually tires which turned out to be trickier than I had ever imagined.  I had to retool my air compressor and associated accessories.

We took the motorhome out weekly to drive it around the block, a 7 mile drive on the outer roads along I-10.  I checked the tire pressure before each trip.  I checked tire temperatures regularly at stops along the way.  These practices, along with a timely tire recall, kept our tires in great shape.  We never had a tire incident.

Knock on wood!  At the 2018 RV-Dreams Spring Educational Rally, tire safety came up over and over again.    I learned a few new things to worry about:

  • Load ranges
  • Tire inflation tables
  • Maximum tire load decreases as speed increases
  • Maximum tire load increases with tire pressure to a cutoff pressure
  • Axle by axle and/or wheel by wheel load

My challenge is to figure out what the right tire pressure is for my truck and load subject to a few very important constraints:

  • Manufacturer maximum tire pressure
  • Self imposed maximum truck speed
  • Actual load and how cargo is loaded

Tire Inflation Tables

The new truck came with Continental HSR 225/70R19.5 commercial truck tires which appear to be typical medium duty truck tires.   Like most tire manufacturers, Continental provides Load/Inflation Tables (also known as tire inflation tables) for the tires they sell.

Tire Load Range

Load range information can sometimes be found on the tire.

Load Range G
Load Range G

These tires have a G load range meaning this is a 14 ply (or equivalent on the tread) tire.

Maximum Tire Pressure

The manufacturers rated maximum cold tire pressure for this tire is 110 PSI and is documented in the tire inflation tables as well as imprinted on the tire itself.

Inflation Table Graphs

The following graphs show how speed affects the tire load carrying capacity.  The two graphs show that as speed increases, the tire’s load carrying capacity decreases.  The information in the graphs comes from the manufacturer’s tire inflation tables.

Why two graphs?  The same tire in a dually has a different load capacity than a single tire.   Dually tires tend to get warmer (or hotter) than single tires.  Heat dissipation in tires affect tire life and load carrying capacity.

An important bit of dually trivia – tire spacing.  There is a minimum recommended spacing between mounted dually tires.  If the spacing is too small, heat dissipation is disrupted and the tires get hot increasing the likelihood of tire failure.  When dually tires touch, they overheat rapidly and are unlikely to last very long.

Single Tire Inflation Graph
Single Tire Inflation Graph

Tire load capacities decrease with decreasing pressure.   Tire load capacities decrease with increasing speed.  Per tire, dually tires carry less load than single tires.  However a dually (consisting of two tires) carries close to 1.8 times a single tire.

Dually Tire Inflation Table
Dually Tire Inflation Table

Truck Manufacturer Tire and Loading Information

On the driver’s door jam, there is a sticker showing the truck’s cargo carrying capacity and the recommended cold tire pressure for front (single) and rear (dually) tires.

Ford F-450 Tire and Loading Information Sticker
Ford F-450 Tire and Loading Information Sticker

Next to the above sticker is another sticker providing the truck’s Gross Axle Weight Rating (GAWR).

2017 Ford F-450 Gross Axle Weight Rating Door Jam Sticker
2017 Ford F-450 Gross Axle Weight Rating Door Jam Sticker

Front GAWR is 6,000 pounds.  Recommended tire pressure is 90 PSI.  At 65 MPH and 90 PSI, the load carrying capacity of one tire is 3,450 pounds.  Two tires on one axle gives 6,900 pounds which is above the front GAWR.  However, at 75 MPH, the load carrying capacity for one tire drops from 3,450 to 2,917 pounds.  Two tires drops from 6,900 to 5,834 pounds.  This is below the front GAWR by 166 pounds.  If the truck front axle is at its maximum capacity, the tires at 65 MPH are fine but overloaded at 75 MPH.  This is a problem.   Increasing the tire pressure to 100 PSI brings the 75 MPH weight rating up to 3,203 pounds for a single tire.  This brings the axle up to 6,406 pounds which is 406 pounds over the front GAWR.

Rear GAWR is 9,900 pounds.  Four tires support this axle.  Each tire needs to be able to support at least 2,475 pounds for this axle:  9,900 ÷ 4 = 2,475.  The following image graphically shows the effect of speed and tire inflation on the ability for tires to support the GAWR.

Speed and Tire Inflation Effect on GAWR
Speed and Tire Inflation Effect on GAWR

With the truck fully loaded and the tire pressure set at the manufacturer’s recommended values, driving the truck at 75 MPH or higher risks damage and possibly failure in the front tires.  However, when the truck is unloaded, the truck is a little bouncy at the recommended tire inflation.  The truck is even more bouncy (irritatingly so) when all tires are inflated to 100 PSI.  Unloaded, there is the possibility that drivers can loose control on some (uneven) road surfaces due to the higher inflation pressure.

The truck camper is mounted on the truck 99.9% of the time.  Little opportunity for driving unloaded so very little chances of loosing control while driving at the higher inflation pressures.

Texas state highway speeds may be higher than other states.  Many Texas highways have 75 MPH posted speeds.  Some Interstates and toll roads have 85 MPH speeds.  When loaded, without proper tire inflation, it may not be safe to drive the speed limit.  Even with maximum tire inflation, it may not be safe to drive the speed limit at 85 MPH.

Truck campers don’t exactly enhance the aerodynamics of full size pickups.  In high winds (35+ MPH), not uncommon across the desert southwest, with truck speeds above 60 MPH,  campers sometimes experience flapping.  The front of the camper lifts off from the truck a tiny bit and then drops back down.  In calm winds, at 75 MPH the truck controls feel a bit sluggish.

A 65 MPH top cruising speed in ideal driving conditions seems reasonable so long as the speed limit is not exceeded.  Having the ability to move faster when warranted (being chased by the devil himself) is prudent.  Proper (higher) tire pressure can ensure that higher speeds are safely attainable when needed.

Abnormal tire wear patterns can be caused by both under and over inflation as shown in the figure below.  Under inflation causes wear on the sides of the tread.  Over inflation causes wear on the middle of the tread.  Getting the pressure right means even wear across the tread.

Effect of Inflation on Tire Wear
Effect of Inflation on Tire Wear

Based on the information presented which is specific to our truck, taking into account our tires, load, driving habits and preferences, we keep our front and rear (dually) tires at 100 PSI. While it might be arguable that the rear dually tires are over inflated for the load, I like all my tires to be at the same inflation value so that all tires have the same contact with the road.

Hope to see you on the road ahead!

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