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December 8, 2011

Plus Sizing the Right Way

Photos by Ulysses Ang
Large diameter wheels and low profile tires are everywhere now from car show rooms, motor shows and even the race track. So whether the vehicle came from the assembly line or was upgraded after it was delivered, Plus Sizing probably played a role in its tire size choice. Plus Sizing allows tires and wheels to make a statement while providing functional improvement.

Plus Sizing isn’t new. In fact, it dates back to the 1970s when several tire options were given to car buyers who could upgrade their rides from relatively narrow 13-, 14- or 15-inch wheels and tires up to wider 14-, 15- or 16-inch combinations. Going up an inch (Plus One), two (Plus Two) or even three (Plus Three) are still popular today but the starting point often begins with 15- or 16-inch wheel diameters and grows from there.

Plus Sizing supports the premise that it’s important to maintain the same overall tire diameter whenever changing tire and wheel sizes to ensure sufficient ground clearance, appropriate driveline gearing and accurate speedometer readings. Large changes in overall tire and wheel diameter can alter the accuracy of the speedometer as well as the effectiveness of several key safety components such as the anti-lock braking system (ABS), traction control and vehicle stability system.

Plus Sizing is one of the easiest ways to improve the image of a vehicle. The visual appeal is obvious since spiffy alloy wheels are more attractive than black tire sidewalls and bigger wheels combined with shorter tire sidewalls give a much more powerful image.

At the same time, using tires with shorter sidewalls also quickens steering response and increases cornering stability. Combining them with larger diameter wheels makes it possible to properly maintain the overall wheel and tire diameter, keeping odometer and speedometer changes negligible.

Of course, Plus Sizing also has some pitfalls. The biggest risk stems from accidental encounters with potholes, curbs and debris on the road. Low profile tire sidewalls can be pinched more easily between the road and the rim because shorter sidewalls cannot accommodate impact as well as taller sidewalls. Once a vehicle has been ‘Plus Sized’, the driver may have to go around obstacles rather than run over them.

Additionally, wider tires tend to float on loose surfaces and cannot process water as quickly as narrower tires. This reduces wet weather traction and offers less resistance to hydroplaning when driving on water-soaked roads.

Finally, going towards the extreme end of the Plus Sizing applications, especially for pick-up trucks and sport utility vehicles (SUVs), can result in significantly heavier rims and tires than the vehicle’s Original Equipment or O.E. set, resulting in longer stopping distances as well as increased suspension and brake wear.

The general rule of thumb for Plus Sizing is to increase tire width by 10 millimeters and decrease the sidewall height by 5 to 10 percent for each 1-inch increase in wheel diameter. There should be less than a few centimeters of difference in the overall diameter of the new wheels and tires to result in negligible speedometer variance.

Modern passenger car tires feature size designations that indicate the tire’s dimensions in a combination of metric, mathematical and English systems. This unusual combination of millimeters, percentages and inches is a by-product of the evolution of global tire specifications and it provides the ability to calculate or estimate the basic tire dimensions. For example: 225/45R17.

The first three numbers (225) are the tire’s indicated section width in millimeters, measured from sidewall to sidewall. In this case, it’s 225 millimeters.

The second pair of numbers (45) is the tire’s aspect ratio or profile. This is a ratio of the sidewall height to section width. The section height’s measurement can be calculated by multiplying the section width by the aspect ratio. The answer will be the height of one sidewall. In this case: 225 mm x 0.45 = 101.3 mm.

The last number (17) is the diameter of the wheel in inches.

To calculate the overall diameter of a tire, the sidewall height must be multiplied by 2 (remembering that the tire diameter is made up of two sidewalls—one above the wheel and one below the wheel, touching the ground) and add the diameter of the wheel: 101.3 mm + 101.3 mm + 431.8 mm (that’s 17 inches in millimeters) = 634.4 mm. 634.4 mm is this wheel/tire combination’s overall diameter. This is also the number you have to look when Plus Sizing.

For example, if you’re opting to Plus One (in this case, going 18 inches), you’ll have to look for a tire with a sidewall height that’s approximately 634.4 mm – 457.2 (that’s 18 inches in millimeters) divided by 2.  In this case, you’ll have to go with a sidewall height of 88.6 mm.

Going back to Plus Sizing’s rule which is to increase tire width by 10 millimeters for every inch in diameter increase, you’ll have to look for a tire that’s 235 mm in sectional width that still offers the 88.6 mm sidewall height. To get that figure, you have to divide 88.6 mm by 235 mm. This equals to 0.37 or an aspect ratio of 37. Since tires are only available with aspect ratios in increments of 5, you can settle for a Plus Size tire of 235/40R18.

Double-checking, the overall diameter of this new tire is 94 mm + 94 mm + 457.2 mm = 645.2 mm. It’s a bit taller but at least the difference versus the O.E. wheels and tires is just 1.7 percent which is pretty negligible. In comparison, a 235/35R18 will yield a 2.01 percent difference. Take note though that drivers working with close fitment tolerances (such that those running on lowered rides) should exercise caution as tire dimensions are often different than the tire’s actual physical size and that the dimensions can actually vary between tire brands and models. In certain occasions, you may experience ‘rubbing’ against suspension components or even the fenders. In times like these, you may have to roll or alter your fenders to accommodate your new set of wheels and tires.

Paying careful attention to Plus Sizing plays a vital role in keeping your vehicle safe and roadworthy while making you look different and stand out from the rest of the crowd.

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