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Wheels and tires affect car's gas mileage

After-market wheels and tires are popular ways to personalize your car or truck, but there is much more to consider than simply which wheel and tire combination looks best.

When engineering your vehicle, the manufacturer invested time and money determining the ideal wheel size and tire type. Often a tire manufacturer works hand in hand with the vehicle manufacturer to concoct a new tire or tread design to meet the manufacturer's specifications for that vehicle. The Environmental Protection Agency establishes the miles-per-gallon, city and highway, numbers included on the window sticker; these numbers are based on the vehicle's original-equipment wheels and tires. Replacing the original-equipment wheels and tires can affect handling and fuel economy.

The Transportation Research Board says 200 million replacement tires are purchased each year at a cost of $20 billion. You can expect to replace the tires on your vehicle every three to five years. It's big business. Although consumers don't typically base their decision to change their wheels and tires on fuel economy, a little research could improve mileage.

Rolling resistance is the key to fuel economy. Stationary objects want to remain stationary. City fuel economy is lower than highway fuel economy because the engine must generate extra power to get your vehicle moving every time the light turns green. Generally, it's easier to get a smaller wheel and tire package moving than a larger one, thus less power is required. Less power required translates into improved efficiency and better mileage. Once moving, however, the engine must work harder to make that same smaller wheel and tire package cover the same distance as the larger one. For better fuel economy when cruising, the larger package would seem to have an advantage.

These assumptions are based on the smaller package weighing the same as the larger package. Steel wheels weigh more than alloy wheels of the same size. Consequently, replacing a 16-inch steel wheel with a 17-inch alloy one could actually reduce the weight and improve overall fuel economy, albeit marginally. If all this noodling is giving you a headache, you are not alone.

I spoke with Kurt Berger, manager of consumer products engineering at Bridgestone Americas Tire Operations, to bring some clarity to the issue. His advice for anyone thinking about moving up in wheel size (plus-sizers, he calls them) is to compensate for the larger size with a lower profile tire. The goal being to ensure the diameter of the replacement wheel and tire package has the same diameter as the original equipment. "If you go up one inch in wheel size," he explained, "you have to compensate by reducing the height of the sidewall."

Reducing sidewall height isn't the only consideration. A shorter sidewall means the tire has a lower load capacity. To maintain the load capacity of the original equipment, the replacement tire must now be wider than the original. This isn't math you need to do. The load capacity is printed on your tire sidewall and is listed among the product specifications of any tire you might buy.

Keeping the diameter of the new wheel and tire package equal to the original-equipment package, should result in very little impact on fuel economy. Generally, the lower profile of the tire should improve fuel economy, but not by enough that you will notice.

What will have an impact on rolling resistance and consequently fuel economy, according to Berger, is inflation levels. A tire under-inflated by eight or nine psi can increase rolling resistance by 20 percent. "The best thing a customer can do is keep the proper inflation," he said. "Check it once a month."

Tread design can also increase rolling resistance: The deeper the tread, the greater the rolling resistance. Off-roaders who put all-terrain tires or tires with a mud pattern on their vehicle will notice some drop in fuel economy because of the deep tread pattern of those tires. What's more, "A worn-out tire has less roll resistance than a new tire," Berger notes.

The bottom line is that changing wheels and tires is a more complex issue than mere aesthetics. It can alter fuel economy and even the handling dynamics of your vehicle. A little time spent researching before buying will produce more favorable results.

Here are this week's reader questions:
  • Wheels and tires affect car's gas mileage
  • Where's my car-payment bailout?
  • Don't renegotiate new car contract

 

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