The string of numbers on a tyre's sidewall tells you exactly what fits your car. Get it right and the car grips, stops and wears evenly; get it wrong and you compromise safety and fuel economy.
A size like 205/55 R16 91V breaks down as: 205 is the width in millimetres, 55 is the sidewall height as a percentage of the width, R16 is the rim diameter in inches, 91 is the load index and V is the speed rating. Match the size on your car's tyre placard — usually on the driver's door frame — not just what's already fitted.
Rotate your tyres every service and check pressures monthly. Even the best tyre wears out early if it's driven under-inflated.