Plus Sizing is a concept designed to achieve enhanced vehicle performance over a base original equipment fitment. There are two main benefits for “inching up.” First, for many customers it is strictly cosmetic. A larger wheel simply looks better. Second, the wider section widths and lower profiles realized from the "Plus" concept yield a wider footprint that increases the vehicle’s steering response and overall cornering force. As the example in the chart indicates, increasing rim diameter, rim width and tire section width while decreasing aspect ratio gives the customer a higher level of handling with no change in gearing or speedometer accuracy.
In determining proper plus sizing fitments, there are several basic principles that must be understood and satisfied.
Historically, the idea of keeping the same overall tire/wheel diameter was related only to maintaining the same gearing and accurate speedometer readings. These two issues are still important, but there are even more critical reasons for maintaining the same overall diameter in modern vehicles.
First of all, virtually all currently marketed vehicles include anti-lock braking systems or ABS, ABS systems work by monitoring relative wheel speeds. Large changes in the overall diameter of the tire/wheel assemblies can alter the effectiveness of certain ABS systems.
Many vehicles also have traction control systems as well as ABS. Once again, changes in diameter can have an effect on these systems. In conjunction with ABS and traction control, stability control systems are also becoming more common. These systems monitor cornering behavior and selectively brake certain wheels to reduce understeer or oversteer, helping the driver more easily maintain control of the vehicle.
Once again, it is important to maintain the overall diameter of the tire at or near the O.E. fitment. Two other issues can also be affected by changing tire diameter. The first, called OBD II, is related to vehicle emissions. The other has to do with the fact that many of today’s vehicles have electronically controlled automatic transmissions. These transmissions use vehicle speed as well as other inputs to determine shift points. If vastly different diameter tires are fitted, not only will the speedometer be inaccurate, but the transmission-shifting pattern will be altered, resulting in less than optimum performance.
Make sure the tire/wheel combination fits without any clearance problems. Check for clearance between the wheel, brake, and suspension components as well as the tire and wheel well areas. Clearances also need to be checked on the road during hard cornering and tight turning.
The chart below is an example that illustrates the effectiveness of Plus 1 and Plus 2 options.