Optimum, appropriate or required inflation? What’s on a tire placard?

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RV Tire Safety

with RV tire expert Roger Marble

I have seen people use a variety of words to describe the inflation numbers on vehicle tire placards. These numbers are arrived at by different methods for different vehicles.

For cars and light trucks (1/2 ton), tire pressure is almost always arrived at based on on-vehicle testing and evaluation by the engineers of the vehicle manufacturer in conjunction with the tire company engineers. There will be a number of evaluations done using prototype vehicles and requests will be made to the tire company to adjust ride, handling, noise and a dozen other objective and subjective qualities of the tires. Eventually, the desired compromise is arrived at with one of the variables being the placard inflation.

For most cars, the inflation is significantly higher than the normal expected GVWR (gross vehicle weight rating). Many cars have reserve load of +20% to over +30% of the normal vehicle loading. You will note that the tire placards for passenger vehicles do not show GAWR (gross axle weight rating) or GVWR.


Lightweight trucks (1/2 ton) will normally be treated more like a car, but as you move to the heavier vehicles federal requirements kick in and by the time you get to HD 3/4-ton and certainly 1-ton units the GAWR and GVWR ratings become a higher priority.

RVs have different DOT requirements and also different tire placard requirements. Most RVs have the inflation specified based on just one criterion – that is load capacity.

DOT requires that the inflation specified to provide the load capacity of the tires, when inflated to the specified level, be able to support half the GAWR.

RVIA in 2017 upped their requirement to require it to be capable of supporting 110% of the GAWR (IMO this is better but not enough).

So the bottom line is that for most RVs the inflation is basically the level needed to support 100% to 110% of the GAWR. But many RVs are not loaded to that level, which means you can lower the inflation to achieve a better ride as long as you don’t end up with overloaded tires.

DOT doesn’t trust the average driver to know the actual weight of the RV. They have done studies and discovered that a significant portion of the population thinks they are OK but actually have tires in overload. So the DOT doesn’t publish alternative inflation guidelines and RV companies follow what the government does.

It’s just that simple.

 

Read more from Roger Marble on his blog at RVtiresafety.net.

 

 


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