Plus sizing is most probably the easiest and cheapest way to improve the performance and the appearance of your vehicle in one foul swoop. In the example below, a 15×7 wheel with a 205/65R-15 tyre is considered the O.E. (original equipment) size. Converting to a plus one size would mean increasing the wheel diameter by one inch (16×7.5) and selecting an appropriate tyre to fit (205/60R-16). Likewise, moving to a plus two fitment would result in a 17×8 wheel and a 225/50R-17 tyre. It is important to note that although the wheel diameter is increasing, the overall diameter of the tyre remains consistent. This is to ensure that the performance characteristics of the vehicle are not compromised in any way. If you do change the overall diameter of the wheel and tyre combo it can have a negative effect on the comfort, the fuel consumption, the performance and the handling of the vehicle. It can also affect many of the electronic components on the more modern vehicles in today’s market, many of them use the tyre diameter to indicate if a tyre is losing air pressure as well as to correctly operate the ABS Braking system.
Two things happen to the tyre to increase performance when moving into plus sizes. First, the tyre becomes wider due to an increase in section width. This provides a larger footprint and more contact with the driving surface. Second, the aspect ratio is lower, resulting in a shorter sidewall. The combination of these changes offers better lateral stability and increased steering response. Basically for every inch that you increase in rim size you need to drop an inch in the tyre diameter. The O.D calculator below will help you in that it will calculate the diameter of the tyre that you intend fitting. In passenger cars it is recommended that the difference in diameter never exceeds 10mm. With SUV’s or 4X4 vehicles there is a lot more space available under the fender and you can increase the diameter substantially more, especially if a lift kit is fitted to the vehicle.