Tire pressure monitoring systems have allowed us a peek into the secret life of tires, and revealed some astonishing details. Several recent studies have shown, for example, that tire pressure can deviate considerably between two tires, one exposed to direct sunshine, the other on the shady side of the truck. You'd be like a dog chasing its tail if you tried to level out all the tires on that truck. And by evening, the pressures will have all changed again.

Top up a tire inside a warm shop in the winter and then drive outside and everything changes again. Inflate a cold tire outside and then bring it inside and you'll have a whole different set of numbers. It's worse than herding cats.

One of the Task Force sessions from the S.2 Tire and Wheel Study Group at this year's Technology and Maintenance Council meeting in Nashville presented some interesting findings on tire pressure maintenance. Case Studies for Proper Tire Inflation Maintenance, chaired by Al Cohn of Pressure Systems International, has yet to go to balloting, so reporting directly on the discussion would be inappropriate, but the story that emerges from the session is interesting indeed -- and there's plenty of other data that tells a similar story.

Even well-managed tire inflation pressure can be all over the map at different times of the day and in different operating conditions.

"Tires are not going to be perfect all day long," Cohn noted. "You settle for 100 psi Monday morning at 8 am when it’s 70 degrees, but the pressure will change significantly over the course of 24 hours."

Tires are designed to account for these fluctuations, so that’s nothing out of the ordinary. Problems could arise, however, if you are not doing your pressure checks properly, or if you wind up inadvertently compensating for some of the natural changes that are going to occur.

Dramatic variations in pressure

Josh Carter, co-founder and CEO at Aperia Technologies, maker of the new Halo self-powered, all-wheel tire inflation system, did some thorough research on tire inflation pressure while developing the product. He found, not surprisingly, that human error can lead to some dramatic variations in pressure, especially with inside duals.

"We used internal pressure sensors in our testing phase, but we still had fleets do manual pressure checks so we had something to compare," he says. "We found variances of up to 10 psi between the manually indicated pressure and what we saw from the tire pressure monitors."

Carter says it was probably a combination of badly calibrated tire gauges and the technician not getting the gauge to fit snuggly onto the valve stem. He says the variations were more pronounced on inside tires.

Carter's research also found that tires exposed to bright sunshine will heat up, with the elevated temperature causing a corresponding rise in pressure.

"We saw variations of as much as 11 psi on the sunny tires compared to the tires on the shady side of the truck and on the inside dual on the sunny side," he says. "That's not a huge cause for concern except if a technician is checking tire pressure and lets a sun-exposed tire go with what appears to be a normal reading. It could actually be about 10 psi underinflated."

He also found that some pressure checks were done while the tires were still technically warm, which would show an elevated pressure over a cold tire, possibly leading to a false sense that tires were correctly inflated.

Cohn notes that it takes about 20-30 minutes fully loaded at highway speed to come up to what is considered hot inflation pressure (about 15 psi above cold pressure), while it can take as long as four hours for a tire to cool for an accurate cold pressure check. Cohn also noted 70 degrees is considered the normal ambient temperature for a cold pressure check. Naturally, if it's 40 degrees outside, or 90 degrees, your pressure readings will be up or down about 2 psi for every 10 degrees over or under normal.

That's what we mean when we say maintaining tire pressure is like a dog chasing its tail.

While you will probably never succeed in getting all the tires on a truck at exactly the same pressure for more than a few hours at time, there is cause for concern about letting the pressure slip too far from normal.

TMC's RP 235A notes, for instance, that an inflation mismatch of greater than 5 psi can result in the two tires being significantly different in circumference, and that has real impact on tire wear -- especially on the tire with lower pressure.

RP 235A describes all the pitfalls of unequal pressure in duals, as well as the understood risks of running underinflated tires.

Relief from Worry

Despite the three big concerns stemming from incorrect inflation -- fuel economy, tire wear and blowouts -- fleets can rest easy if they have a few basic checks and balances in place with their tire inflation management process.

• Do regular tire pressure checks under similar conditions, i.e., when the tires are cold, when ambient temperature is about the same, and use calibrated tools.

• Tools exist or can be easily fabricated to fill several tires from a single manifold linked to a calibrated gauge. Filling all the tires at once will give you accurate and balanced pressure between all tires.

• Automatic inflation systems will maintain a preset pressure or dual-tire pressure equalizers can maintain equal pressure between the two tires.

• Tire pressure monitoring systems provide fleets and/or drivers with real-time inflation pressure alerts. Temperature compensating systems improve the accuracy of the reading, screening out "normal pressure" readings even when the tire is hot, which could leave it under-inflated when cold.

While tire inflation pressure is worth worrying about, you'll never be able to stay on top of it totally without some outside help. Technology such as automatic tire inflation systems and tire pressure monitoring systems can help even out the swings in pressure, or at least bring them to your attention.

Simply doing more frequent pressure checks can help, but they have to be done under the proper circumstances, such as when tires are cold and when wild swings in ambient temperature won't skew the readings.