Wheel upgrades should start with shop visits

As with most things high fashion -- including wheels -- appearance usually comes at a price. At risk is a lot of money, a lot of time and, potentially, your safety.

The aftermarket began the practice of "plus one" and "plus two" sizing, bumping up rim diameters without changing the overall diameter of the tire-wheel combination. This made it possible to display more wheel and less rubber while still neatly fitting everything under the fenders.

Auto manufacturers soon joined in, offering larger wheels and short-sidewall tires -- even 20 inches or larger -- to cash in on the craze.

But while that practice is common for limited-use customized street cars and exotic vehicles, the trend has found its way to the lower end of the new-car market. That means they're available on a mass scale. For example, a Chevrolet Cobalt can be equipped with 15-, 16- or even 18-inch wheels and tires. That's a large operating range and requires careful parts integration and suspension tuning to make it all work. It's not a process that should be left to an amateur.

Doug Parks, who was chief engineer for the Cobalt and the Pontiac Solstice sports car, said that there's a lot to consider when going bigger.

"When you're talking about ride and handling, if you don't have a great tire, you're starting from behind," he said. "We gave up two-tenths of a mile per gallon because of higher rolling resistance (by choosing wider tires), but (handling) was the priority."

"Plus one" can be explained as going up 1 inch in wheel diameter and 10 millimeters (to 255 from 245, for example) in section width (the distance from one outside sidewall to the other) without changing the overall rolling diameter of the whole combination. (See the infographic at www.wheelbase.ws/wheels). That means the tire sidewalls are shorter. "Plus two" means a 2-inch larger wheel diameter and a 20-mm wider tire. Again, the sidewalls take up the slack.

Maintaining the overall height -- or rolling diameter -- is critical to maintaining speedometer and odometer calibration, transmission and final drive gearing and suspension design and tuning.

The hard reality, literally, of adding bigger rims is that, as good as they might look, they're historically the result of the desire to have wide tires with stiff, short sidewalls that improve steering performance and high-speed vehicle stability.

Low-profile tires actually require the use of larger-diameter wheels, and since they're made for performance, they normally sacrifice ride quality, tread life and, to varying degrees, four-season dependability.

That doesn't seem to faze some owners, while others perhaps don't realize that low-profile tires aren't ideal in all weather conditions.

Generally, if you own a vehicle with 50-series or lower-profile tires and are not storing it for the winter, you'll want to at least store the wheels and tires and bolt on less-expensive steel wheels shod with taller-sidewall winter-specific rubber. Bridgestone-Firestone says the main problem is that wider tires hamper the ability to cut through the snow to find the grip. The company actually recommends minus-sizing for winter tires, or going to the smaller diameter -- and narrower -- than original equipment sizes.

If an upgrade is in the cards, the best place to begin is with a shop that specializes in engineered packages, whether at the dealer level for your vehicle, or at a reputable tire/wheel shop. Regardless, the more you know, the better the odds of making the correct -- as in safe -- decisions.

Richard Russell is a feature writer with Wheelbase Communications. Drop him a note online at www.wheelbase.ws/mailbag.html. Wheelbase is a worldwide supplier of automobile news, reviews and features.