Depending on what a tire's intended use is, the speed rating will vary. In theory, a given car can use any speed rated tire, but performance will be affected.
Speed rating relates to the maximum speed a particular car can reach. The lowest speed rating is an L-rating, which is 75 mph and is used on light trucks and off-road vehicles. Most family sedans use an S- (118 mph) or T-rating (118 mph), while many sport sedans use a V-rating (149 mph). The highest speed rating is Z (99Y), which is rated for speeds in excess of 186 mph.
The biggest affect speed ratings have concerns the theoretical top speed of the vehicle. If your vehicle is not a high performance vehicle and is never driven at high speeds, a lower speed rated tire may used safely.
Generally speaking, a given vehicle should use a tire with the same speed rating for which its manufacturer designed it. If you use too low of a speed rating on a high performance car that was designed for a high performance tire, the handling will likely not be optimal and the vehicle may not be able to be driven the way it was intended. However, using a higher speed rating on a car that generally uses a lower speed rated tire may improve the vehicle's handling.
