The speed rating indicates the maximum speed at which the vehicle can travel under maximum load. Failure to observe this value can lead to reduced comfort and, in extreme cases, even to the tyre blowing out while driving.
Before you decide to change size, make sure that the indexes (speed and load capacity) are in accordance with the manufacturer's recommendations. You will find the recommended ratings in the vehicle handbook.
You will read the speed index from the tyre size. Example: 195/65 R15 91 V, the letter "V" indicates a speed index of 149 mph.