Bottom Line
Maintenance Q&A
Be wary of overinflated ‘advice’

By Paul Abelson
Senior technical editor


Q: In a discussion at the truck stop, a driver mentioned that he heard that tires underinflated by 20 to 30 percent lower fuel economy up to 5 percent. “It stands to reason,” he said, “that increasing pressure 20 to 30 percent will save fuel.” He says he runs his tires 30 psi over recommended – steers at 140 psi and drives at 130 psi. He says his fuel mileage has improved, and he goes farther between fill-ups. What do you think?

A: I think there’s a great deal of misinformation at truck stops – not in the service bays, but in the “drivers only” section. Just to be sure about my opinion, I checked with one of TMC’s wheel and tire experts, Al Cohn of Pressure Systems International.

Al, a TMC Silver Spark Plug award winner, also chairs SAE’s Commercial Vehicle Maintenance Committee. According to him, your truck stop friend is driving a ticking time bomb, one that is costing far more than it is saving.

His wheels and tires are being stressed beyond their design limits. Most big rig tires are safety rated to 120 psi maximum. Wheels are rated to 130 psi. He could go for months or years, then hit a pothole and have a catastrophic blowout or wheel breakage.

Air pressure 30 to 40 psi over designed limits rounds out the tire. That reduces contact with the pavement and causes excessive center tread wear. The tires will also bounce excessively, causing even more irregular wear. He must feel the bouncing as a harsh, uncomfortable ride.

Each tire manufacturer has load-inflation tables for each tire on its Web page. You’ll get the best combination of fuel economy, tire life and ride quality by adjusting tire pressure to do the job the tires are called on to do.

You can lose 2 percent or more fuel economy not keeping your tires aligned. Out-of-line tires scrub diagonally across the pavement, wearing out prematurely and using energy to create the wear.

But, please, no 140 psi inflation.

Q: The alternators on my 2006 Kenworth keep going out. I keep replacing them and they fail again. The rebuild shop says that I’m installing them wrong because the bearings come back shot. I follow the procedures in my shop manuals to the letter. I told the shop if the bearings go bad, they’re probably counterfeits like the ones you wrote about. Who is right?

A: The problem may not be with the alternator bearings, but with your belt tensioner. At a recent TMC meeting, one fleet reported problems with 18 tensioners out of a purchase of 20. Five of them were brand new.

Underhood heat from the EGR system may be the culprit. It can cook some of the lubricant and affect polymer parts. They cause the tensioner to bind, causing the belt to slip over it. The slippage creates heat from friction. The heat transfers into the belt, then to the alternator pulley. That heat is added to underhood heat from the EGR system. It can get hot enough to destroy the alternator bearings.

Check the belt. If it is glazed or shiny, it is probably slipping on the tensioner. If there is side wear, there could be pulley alignment problems. Not all manuals call for checking belt and pulley alignment. Belt tension must be checked with the engine off, so you can’t gauge the dynamic action of the tensioner. That’s why you have to examine the belt.

Like a tire, a belt will wear down. If the automatic tensioner isn’t working properly, it will be undersized and will slip, adding more to the heat in the alternator pulley. The V-section sides do the driving. If they wear too much, the belt will ride up on the points of the Vs, losing traction on the drive and driven pulleys. LL


Paul Abelson can be reached at