We often receive questions from customers wondering why tires with a certain speed rating are installed as original equipment on their vehicles. One customer asked, “Can you explain why a car like a Nissan cube needs tires that have a speed rating of over 200 km/hr is required?”
During the design and building phases of a new vehicle, many automobile manufacturers work in collaboration with tire manufacturers. Tires play an intricate part of a vehicle’s performance and there are certain criteria they are trying to develop, such as wet and dry handling, braking, road noise, ride comfort, steering feel, fuel economy and many others. In some cases tire engineers are assigned to work right alongside chassis and suspension engineers during vehicle development. In most cases after the development stage, tires are tested by both the tire and vehicle manufacturer to ensure they meet or exceed the specific criteria desired to maximize performance. The tire tests can include wet handling, wet braking, high speed stability and durability, rolling resistance, dry grip and braking, road noise, hydroplaning resistance, uniformity and others. In essence, a V-speed rated tire is not just a speed issue but rather a vehicles performance issue.
Let me also explain a bit about “Speed ratings” or “Performance Ratings”.
The Tire Industry Association (TIA) defines a speed symbol as “A letter(s) that indicates the maximum speed a tire can safely operate for a given load index under specific service conditions. Speed ratings are based on laboratory tests under controlled conditions, so the actual operating speed may be less than indicated speed symbol due to inflation pressure, load, tire condition, wear, driving conditions, or other factors.”
Speed ratings or performance ratings are based on laboratory tests where the tire is pressed against a large diameter metal drum to reflect its appropriate load, and run at ever increasing speeds (in 6.2 mph steps in 10 minute increments) until the tire's required speed has been met. It is important to note that speed ratings only apply to tires that have not been damaged, altered, under-inflated or overloaded.
Speed ratings are a durability standard defining a tire’s ability to withstand or control heat and growth. Higher speed rated tires are better equipped to dissipate heat, they offer better cornering, gripping and braking, therefore, better overall handling. Speed ratings are based on three elements.
- Tread Stability
- Rubber Compound
Toyo recommends that you never downgrade speed rating for your vehicle (with the exception of winter tires) and never mix tires with different speed ratings on the same vehicle. Doing so could potentially compromise the vehicle’s handling and your safety.
A “Speed rating” or “Performance Rating” is based on a tires ability to withstand or control heat. Going down a speed rating for a winter tire is acceptable because it is assumed that speeds will be reduced and the temperatures are cooler. Therefore, when the temperatures start to climb and consistently stay above 7 degrees Celsius, it is time to switch back to all-season or summer tires. Winter tires used to not have speed ratings but it was understood that they used to be a Q-rated tire. However, in recent years, many tire manufacturers have designed and manufactured winter tires with higher speed ratings. Toyo passenger winter tires have speed ratings that range from a S (180 km/h) and T (190 km/h) to and H (210 km/h).
Written by Alexandra Leith