Calculate Tyre Dimensions for Plus Sizing/Up-Stepping
What is Plus Sizing?
Plus sizing or up-stepping is the practice of replacing the wheel with a larger one and mounting a lower profile tyre while maintaining the same (or approximately the same) diameter and circumference as the original tyre. These dimensions must be maintained in order to prevent considerable changes in speedometer accuracy, torque and traction control.
Plus x sizing means increasing the wheel diameter by x inches, so plus one sizing means increasing the wheel by one inch. A special case is plus zero sizing which means switching to a wider tyre without replacing the original wheel.
Changing to a wider tyre requires reducing the aspect ratio (which is the second number in the sequence of numbers that describes the tyre's size e.g. 195/65 R15). Since the aspect ratio is the ratio of a tyre's width to its height, it follows that if the width is increased, the aspect ratio should be decreased if the final result is to remain the same—which is the objective of plus sizing.
Why Plus Sizing?
Plus sizing is usually done to improve aesthetics but also performance since original tyres are fitted with respect to, beside performance, noise, abrasion resistance, and comfort. Therefore, they may not be good enough for performance enthusiasts.