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AUTO Q AND A: How does a tire-pressure system work?

Q: I have a tire pressure read-out on my new Jeep Compass and just love it. How do they do it and is it accurate? Also, what would be the preferred RPM to maximize your MPG? I feel like keeping it under 2,000 RPM gets my best mileage?...

Q: I have a tire pressure read-out on my new Jeep Compass and just love it. How do they do it and is it accurate? Also, what would be the preferred RPM to maximize your MPG? I feel like keeping it under 2,000 RPM gets my best mileage?

A: This is a neat feature. Tire pressure monitoring systems (TPMS) first appeared in an automotive application on the 1986 Porsche 959 and gradually increased in popularity on European luxury and performance oriented vehicles, followed by other carmakers worldwide. In 2007 TPMS became federally mandated on all light-duty vehicles as a result of the Ford Explorer/Firestone tire problems. Correct tire pressure also saves fuel, reduces carbon-dioxide production, and improves vehicle handling. The government standards require driver notification should any or all tires fall more than 25 percent below the cold tire specification, although better systems are able to detect variations of as a little as two or three PSI.

TPMS may be of the direct or indirect type. Direct systems employ a sensor within each wheel, and communicate with a central vehicle receiver via radio signals. Sensors may be attached to the inside of the wheel's valve stem or strapped to the wheel's drop-center. Most sensors employ a battery, which in spite of elaborate power saving strategies, will require replacement in perhaps seven to 10 years. Battery-less sensors are also in use, and will become more popular as technology improves. Tire service techs need to be very careful when renewing tires to not damage the vulnerable/protruding TPMS sensors. Various position re-learn procedures, depending on the system, are necessary should a sensor require replacement or battery renewal. Indirect TPMS systems, no longer in vogue, infer tire pressure by monitoring wheel speed information supplied by the antilock brake system. A low tire has a smaller diameter, resulting in a higher indicated speed, relative to its mates. While cheaper to keep, indirect systems require periodic calibration, via a driver reset switch, and typically don't meet the 2007 government standard for identifying four low tires.

Regarding your RPM question, best fuel economy occurs when an engine is running at its lowest comfortable speed. A slower running, moderately loaded engine at part throttle has lower pumping losses than a higher RPM, lightly loaded engine with reduced throttle opening. If driving a manual transmission vehicle, when conditions allow it, early up-shifts, even skipping a gear or two, and running in the tallest gear possible, will yield the best fuel economy. Avoid doing this in hilly terrain or should an impatient driver be behind you. Excessive lugging of the engine (resulting in shuddering or balkiness) can shorten engine life. With practice one can learn to feel the difference between a loaded versus grumpy engine. A lightly loaded larger engine may run smoothly down to 1,200-1,500 RPM, while a smaller engine is happiest above 1,700-2,000 RPM.

Automatic transmission vehicles are calibrated to do all this for you, and grab a lower, more performance oriented gear when greater engine output is desired. Also, variable valve timing/electronic throttle control equipped engines perform sophisticated breathing magic to further reduce throttle resistance, improving efficiency.


(Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose. E-mail under-the-hood@earthlink.net . He cannot make personal replies.)

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