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.
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.
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.
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.
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.
Next to the above sticker is another sticker providing the truck’s Gross Axle Weight Rating (GAWR).
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.
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.
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!
Where did you find the load rating decrease based on speed? Could you share the source data?
My tires are made by Continental Tires. I made a Google search using search terms “continental tire medium radial chart data”.
Results led me to this document – medium-radial-chart-data.pdf.
Inside the document, search for “225/70R19.5”. The table entry provides load carrying capacity (single/dual) by pressure. This table has the following notation in its footnotes – “NOTE: All values are for 65 mph unless otherwise noted, for tire load and inflation at various speeds, see next pages.”
Unfortunately, the above link is missing the “Load Limits At Various Speeds Tires Rated At 75 Mph And Radial Ply Truck Tires Used On Improved Surfaces” section. I searched through the Continental Tire website. I couldn’t find a complete copy of the tire data guide. Tire manufacturers should be providing this information. I’m surprised and dismayed that Continental doesn’t have the data currently available.
Fortunately, I kept a copy of the 2016 “Tire Data Guide.” I’ll email a copy to you. It only covers Continental Tires.
Best of luck figuring this out for your truck and tires.
Thank you for the detailed response! As it happens, I have the same tires on our F450 (Conti Hybrid HD3 in 19.5) and the research in this post has been super useful. I have a copy of the current guide for these tires: https://drive.google.com/file/d/1zU_bBYZd1UZou4eiyPy_uiMFXYs8wfuG/view?usp=sharing
I found the speed rating offset table in the new version of the guide, but it has some interesting wording at the top: “The service load and minimum (cold) inflation must comply with the following limitations unless a speed restriction is indicated on the tire or the manufacturer rates the tire at 75 mph or above.”
(See sections here: https://imgur.com/a/y5D6Eqi)
My HD3s are rated at 87MPH, which is definitely above 75. Physics would tell us that the heat generated at high speeds would require higher pressures for a given load, but that table’s header casts doubt on that requirement.
All of which is to say, I’m a bit confused about whether the speed derating described here actually applies to my tires, or if they are rated to handle the weight at a given pressure all the way up to 87MPH. What do you think?
Thanks for finding the 2020 Continental Tire Guide. Very helpful.
Looking at the Conti Hybrid HS3 225/70R19.5 tire product data on page 56:
There is a column for maximum speed which indicates 87 MPH. Another set of columns for maximum load at inflation for single/dual tire configurations providing maximum weight at a given maximum inflation.
Looking at “Load Limits At Various Speeds Tires Rated At 75 MPH And Radial Ply Truck Tires Used On Improved Surfaces” on page 101 of the guide:
The table for conventional tires gives a variety of speed ranges at various inflation pressure changes. The notes section below the tables only adds to the confusion.
Ambiant temperatures are not mentioned anywhere in the product literature.
My opinion, for what it’s worth: Given that we are talking about conventional tires and the speed range in the load limits table shows no inflation pressure or load changes for 51 through 75 MPH, I believe the best course of action is to assume that the product data table giving maximum load at inflation means at a speed of 75 MPH. This was clearer in the previous 2016 product guide that I have. I’m assuming the tires shouldn’t be driven at speeds over 87 MPH at load for risk of tire failure.
Yes, I am conservative when it comes to tire ratings and capabilities. The lack of clarity in the product data worries me. Stay safe.