S speed-rated tires should never be used on a vehicle that is equipped with H speed-rated tires as it could compromise the vehicle's handling. It is possible to use tires with a higher speed rating, but lower speed-rated tires should only be used if switching to winter tires.
A tire's speed rating is basically the maximum speed at which the tires can travel safely over a given period of time. S-rated tires have a maximum speed of approximately 112 miles per hour and are usually recommended for vans, family sedans and other less powerful vehicles. On the other hand, H-rated tires have a maximum speed of up to 130 miles per hour and are recommended for sports sedans and coupes.