Web Exclusive: Plus-Size Tires 101

Jun 16, 2010

What is Plus-sizing? Plus-size tire selection offers your customers benefits both in terms of appearance and performance.

While maintaining the original equipment tire overall diameter, Plus-sizing allows you to install large rim diameters, wider tires and tires that feature shorter sidewalls.

“Tire purchasers for this segment are split between retaining a near-OEM appearance due to the burgeoning collector car market or installing Plus-size [tires] for an even-greater improvement in style and performance, and to fit the larger aftermarket brake packages which greatly improve the vehicle’s stopping power,” said Joe Jordan with Toyo Tires in Cypress, California, of the Plus-size trend among classic car and truck owners.

From an appearance standpoint, this means that you can feature more wheel and less tire sidewall height, providing a sporty-to-extreme visual statement. From a performance standpoint, this allows your customer to utilize a wider tire (which means a larger tread contact patch for greater traction) and a shorter sidewall. The shorter the sidewall, the less compliant the tire will be in turns where lateral forces are at work. This means quicker steering response and crisper handling.

Keep in mind that this isn’t an invitation to run any size package that your customer might wish. Limitations may include tire size availability, available body and chassis clearance, and inadequate load rating.

Also, even if a Plus-size tire of your customer’s choice is available, he needs to be aware of the consequences of getting too radical in terms of sidewall height. Remind him that, generally speaking, the shorter the sidewall, the stiffer the tire will be, so if couch-comfort is his goal, he can create a less-than-desirable harsh ride by choosing a notably shorter sidewall.

In addition, he needs to be aware of the road hazard potential. After all, the shorter the tire sidewall, the closer the wheel rim will be to the pavement (a Plus-one fitment places the wheel rim ½-inch closer to the pavement; a Plus-two places the wheel rim 1-inch closer to the ground, etc.).

This translates into a less-forgiving environment when encountering potholes and other road obstructions, because the closer the rim is to the ground, the higher the chance of rim damage.

Common Terms

Here are commonly used terms that your technicians need to understand when working with tire and wheel packages in general, and Plus-sizing options in particular, as well as steps to follow when installing Plus-size tires.

Overall Diameter: the outside diameter of the tire when mounted and inflated, but with no vehicle load.

Section Width: Also called overall width, this is the maximum width of the tire’s cross section of an unloaded, mounted and inflated tire (the widest point of the tire when mounted and inflated, but with no vehicle weight).

Free Radius: The radius of the tire/wheel assembly that isn’t affected by load. This is the distance from the wheel axle centerline to the top of the tire tread face.

Loaded Radius: The distance from the wheel axle centerline to the ground, drawn vertically. This is the distance from the vehicle hub centerline to the ground when a tire is inflated and when the tire supports vehicle weight.

Nominal Wheel Diameter: This refers to the size of the wheel applicable for mounting the tire (diameter of the rim seat that will support the tire bead). The bead-to-bead diameter is measured at the inside diameter of the tire, once mounted.

Section Height: The distance from the bead to the tread face.

Loaded Section Height: The loaded radius, minus half of the nominal rim diameter.

Aspect Ratio: This refers to the tire’s section height in relation to its section width, as a percentage. For example, a 60-series tire features a sidewall that’s 60 percent as tall as the tire’s section width, and a 50-series tire will feature a shorter sidewall, approximately 50 percent of the section width. As a formula, aspect ratio can be determined by taking the nominal section height, dividing it by the nominal section width, and multiplying the result by 100. For example, if the section height is 3 inches and the section width is 10 inches, divide 3 by 10 and multiply that by 100. The tire in our example is a 30-series tire.

Tread Width: This is the distance measured from the inner tread shoulder to the outer tread shoulder. Tread width isn’t to be confused with section width, which is always greater than tread width.

Plus-Zero: This maintains the original wheel rim diameter but increases tire section width by 10mm and drops the aspect ratio by five points. This provides a slight increase in tread contact patch, along with a slightly shorter sidewall for quicker steering response and less lateral lean, while maintaining the original overall tire diameter. This is a good idea for the customer on a budget,  since it doesn’t require wheel replacement.

Plus-One: A Plus-one move requires an increase of 10mm in section width, a reduction of aspect ratio of 10 points, and an increase in wheel rim diameter by one inch. If the original setup featured 185/65R14 tires (with overall diameter of 23.47 inches), this would move him into a set of 195/55R15 tires.

Plus-Two: A Plus-two move would require a 20mm increase in section width, a 20-point drop in aspect ratio and a two-inch increase in wheel diameter, as compared to the original package. For example, moving from 185/65R14 would place him into a size 205/45R16 (since that size is, indeed, available).

Plus-Three and Beyond: When considering a Plus-three or greater upgrade, we can’t blindly add a set number of millimeters to section width and drop aspect ratio by a set number of points. Beyond Plus-one and Plus-two, larger rim diameters and the limitation of practically available tire sizes make the old standard formula non-practical. In other words, if your customer wants a Plus-three, you don’t automatically look for a tire that is 30mm wider in section width and 30 points lower in aspect ratio, because that specific tire size may not exist. Instead, whenever moving to a Plus-three or beyond, simply review the manufacturer’s tire sizing chart to find a tire that matches the new rim diameter, maintains the desired overall OEM tire diameter, and matches or goes beyond the OEM tire load index.

Minus-Sizing: Minus-sizing is the reverse of Plus-sizing. Instead of moving to a larger diameter wheel and wider tire with a shorter sidewall, as you would in a plus move, you move to a narrower tire with a taller sidewall. In either case, when Plus-sizing or Minus-sizing, you still must maintain the original overall tire diameter.

5 Steps to Follow When Plus- or Minus-Sizing:

In order to avoid problems with speedometer calibration, ABS, traction control and electronic stability control, the replacement tires should feature the same overall diameter/rolling circumference as the original-equipment tires.

However, it’s widely accepted that a variance of 2-3 percent or less should pose no adverse effects on ABS, traction control or vehicle stability control system operation. This gives your customer a bit of leeway in case the replacement tires of choice don’t exactly match the originals in terms of overall diameter.

When changing tire size, take the following precautions:

1. Make sure that the intended replacement tire will fit onto the intended wheel rim width. Always adhere to the tire manufacturer’s recommendation concerning appropriate rim width for any given tire.

2. Never install tires that feature a lower load rating than that of the OEM tires. Always choose a tire that features an equal or higher load rating.

3. Never use an inflation pressure that’s lower than that recommended by the vehicle maker for its original equipment tires. Also, if the vehicle manufacturer specifies different inflation pressure for front tires as compared to rear tires, always follow the same relative pressure difference. If standard load rated tires are to be replaced with extra-load rated tires, inflation pressure may require a slight boost.

4. Make sure the speed rating of the replacement tires is equal to or higher than that of the OEM tires.

5. Always verify adequate body and chassis clearance of the tire/wheel package whenever you deviate from the OEM tire and/or wheel dimensions. If the customer plans to add tires chains, consider the additional clearance required to prevent chains from contacting chassis or body surfaces.

Plus-sizing also can diminish braking performance because larger-diameter wheels and matched high-performance tires that accommodate these larger wheels typically weigh more than an OEM tire/wheel package. This increase in unsprung weight (and rotating weight) decreases the brake system’s ability to stop the already-heavy vehicle mass. As a result, the brakes are now trying to slow a heavier and larger-diameter package than before.

There are two approaches to this problem-either avoid the issue by limiting the upgrade (wheel and tire size/weight package) or upgrade the braking system to better accommodate the increased mass of the new wheel/tire combo.