The 40 is the height of the sidewall as a percentage of the width of the tyre- tyres are marked 205/40/17 (for example). the 205 is the width of the tyre in millimetres, the 40 is the height of the sidewall is 40% of 205 millimetres and the 17 is the diameter of the wheel in inches... no idea why they mix metric and imperial, but all tyres are marked the same way, with few exceptions. The letter is the speed rating of the tyre, usually H, V or W on modern cars. All PROPER winter tyres must have the snowflake symbol on the sidewall, but M+S tyres have no criteria for how good or suitable they are for winter.