The letters that you see on a tyre - usually after the rest of the information - relate to the maximum speed that a tyre can perform at. Each letter has a different top speed associated with it which will mean that certain cars such as performance cars wouldn't be able to use simple passenger car tyres. In general, winter tyres have lower speed ratings than their summer counterparts. However, an H rated winter tyre can still travel at speeds up to 131mph. If you have any doubts about your current tyre selection, then please consult your vehicles manufacturer handbook to check the speed rating acceptable for use on your vehicle.