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Speed ratings exist to help match the performance of the tire with the performance of the car. They are established in kph then converted to mph (which explains the wonky increments). The ratings are based on lab tests on new, properly inflated tires, so damage, age, and wear can lower the tire’s effective speed rating. Early on, the speed rating was embedded within the tire size e.g., 235/50/VR16), but after 1991 the rating had to be shown in a separate portion of the tire’s description (e.g., 235/50R16 89V, with V being the speed rating).