If you intend to fit winter tires to your car, have a look at minus one and minus two conversions.
If you are planning to fit wheels of a larger diameter to your car, you must equip them with lower profile tires. And if you are thinking about buying lower profile tires, you will also have to buy larger wheels. Why? Simply because the rolling diameter, the diameter of the wheel / tire combination, must remain the same for the following reasons:
- to allow the speedometer and odometer readings to remain accurate;
- to prevent the engine from running at too low an RPM at a given speed;
- to prevent the tires from rubbing in the fender wells;
- to ensure that suspension operation is not adversely affected.
Making a “plus one” conversion means fitting a one inch larger wheel with a suitably lower profile tire to keep the correct rolling diameter, while a “plus two” conversion involves a two inch larger wheel with an even lower profile tire.