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The Full Story on Under-Inflation and Fuel Economy

As the Greenhouse Gas Phase 2 rules are getting fleshed out, automatic tire inflation systems are being considered as a major fuel-saving technology. As it stands right now, the Environmental Protection Agency (EPA) and the National Highway Traffic Safety Administration (NHTSA) will offer compliance credits to any OEM that installs these systems.

According to industry insiders, the decision was based on information gleaned from a Department of Transportation (DOT) study completed between 2008 and 2010. The study revealed that on average fuel economy improved by 1.4 percent when wheel-mounted and valve-stem mounted tire pressure monitoring systems were used.

It’s About More Than Underinflating

There is little doubt that trailer automatic tire inflation systems help in reducing downtime costs associated with flat tires, but what can be said about their contribution to fuel economy?

We’ve all heard of the old adage that 10 percent under-inflation can cause a 1 percent degradation in fuel economy, mainly due to the increased rolling resistance of softer tires. But what exactly does this mean?

You need to isolate whether you are talking about one tire, all 18 tires, just the tires on the truck or just the tires on the trailer. Would all or one of them have to be under-inflated in order to see a 1 percent drop in the truck’s fuel economy? It might be a little bit more complex than that.

The overall fuel economy of your vehicle is depending mostly on the axle position of the under-inflated tire. The fact is, drive and trailer axles contribute more to overall fuel economy than the steer tires. So don’t look there to get the most savings.

What is Recommended?

It’s no secret that most fleets are not very good at tire pressure maintenance. One European study revealed that 12 percent of fleets routinely run under-inflated tires compared to whatever said fleet’s inflation guidelines are.

Another DOT study from 2003 showed that only half of all commercial truck tires were inflated to within 5 psi of the recommended pressure. A full 1 out of 20 were more than 20psi under-inflated and a fifth of dual-tire assemblies differed by more than 5 psi.

So the question remains: What exactly is the recommended tire pressure? There is only one official reference to tire pressure in any industry manuals and that is the tire makers’ load and inflation tables. The information can be found through the Tire Industry Association.

These tables are standards that are consistently maintained and designed to provide guidelines as to which tires can carry what types of loads. Keep in mind that these guidelines speak only to the minimum pressure required to support the load, so tires that are being operated under the guideline are running at below the threshold of their load.

Potential Problems

It can be reasonably concluded that any tire that is run below the minimum standard in the load and inflation guide is technically under-inflated. Under-inflation of this kind can result in tire damage. Whether it be through tread separation or increased flexing in the sidewalls leading to a zipper rupture, running with under-inflated tires is a serious safety issue.

Under-inflation also causes tire flex as they are rolling down the highway. Tire flex not only could damage the tire, but it also affects the truck’s overall fuel economy. Additionally, you may get uneven wear on any number of tires, which could spell additional problems if you are using retreads.

In the end, you’ve got to make the decision that is right for your equipment in relation to the type of loads you are running and terrain you are running it over. While you may realize some savings with under-inflated tires, you’ve got to make sure you adequately weight the risks to the benefits.

In the end, whether you choose to use automatic tire inflation systems or not may not even be up to you. If the EPA and NHTSA go through with the new GHG rules, fleets will have greater incentive to make the switch.

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