Reader D.W. noted, “I am looking at buying a car that offers an option for 20” tires versus the standard 18”. Outside of looks are there any advantages to 20” tires?  Do they provide a smoother ride, better cornering or provide better traction? Also I have been led to believe that for a smoother ride you want as little weight as possible between the road & the vehicle’s springs (tire, wheel, brakes & axle). I would appreciate any insight you have on the subject.”

It’s common for auto manufacturers to offer wheel/tire options with a larger diameter than “stock,” and there are many factors to consider when choosing them.

There is a ride/handling advantage to reducing “unsprung” chassis weight (tires, wheels, brakes), but the difference in 18-inch versus 20-inch wheels is negligible in that quest.

“20-inch” actually refers to the wheel diameter, which, of course requires a “20-inch” tire to fit that wheel.  Since the height of the tire/wheel assembly must be constant for a given new car model, the corresponding design and “profile” of the required tire is actually what creates the factors involved in choosing a wheel size.

That profile is the “aspect ratio” of the tire.  When the wheel size goes up, the height of a tire’s sidewall must be altered to achieve the same overall height of the tire.  The optional 20-inch wheel requires a shorter sidewall dimension than the 18-inch wheel if the two are to have the same overall height (diameter) and circumference.

The larger diameter wheel will be outfitted with a tire having a shorter sidewall, and also a wider tread, because the aspect ratio of the tire on the bigger wheel will be lower.  The aspect ratio is the ratio of the sidewall height expressed as a percentage of the tire’s width.  For example, if the overall diameter of a tire package offered by the auto manufacturer is to be 29 inches, an 18-inch wheel would have a 225 65 R 18 tire, whereas a 20-inch wheel would need a 255 45 R 20 tire.  The tire fitted to the 18-inch rim has a sidewall that is 65 percent of its 225 centimeter cross-section width, and the sidewall of the bigger tire is 45 percent of 255.

The shorter sidewall is inherently stiffer, and having less “squirm,” will improve steering response; the wider tread will also improve traction and cornering adhesion.  Though perceptibly different with each car and suspension design, the shorter sidewall of the tire on the larger rim will also sacrifice some degree of riding comfort.

A less objective consideration when choosing the optional tire/wheel package is cosmetics.  Usually, the larger wheels are offered with premium characteristics such as lighter alloy, chrome finish or aesthetically superior designs.  So, a big part of the decision may depend simply on which wheel/tire you most like the looks of.

Tire tread design is another deciding factor.  On average, the standard sizes have more conventional tread consisting of smaller tread “blocks” with built-in sipes.  For performance, the optional sizes regularly have larger tread blocks to put more rubber on the road, and lack siping.  As a result, if you plan to use the tires as an “all season” choice in the winter, the smaller ones will better fit your needs.  Also, the “fancier” 20-inch wheels may have a finish that is less resistant to the ravages of winter.

Readers may contact Bill Love via email at precisiondriving@spokesman.com.