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The Importance of Tire Speed Ratings

There seems to be a misconception that is very common on the web.

“I’m not going to drive 1** mph so there is no reason I can’t downgrade my tires” .

Part of the attraction here is to purchase a cheaper, longer lasting, lower rated tire… but at what risk?

Vehicle Manufacturers:

When cars are manufactured, they are tested with the size, load and speed-rated tire eventually assigned to the car. The suspension and engine options also effect the recommendations for specific vehicles. Tires with different speed and load ratings handle differently. Maneuvering corners and general handling of the vehicle can be compromised by downgrading the speed or load ratings of the tires assigned to the vehicle. Cars that have higher speed ratings have different performance criteria. Many import manufacturers have tires with higher speed ratings assigned to their vehicles for performance reasons.

Tire Manufacturers:

Tire speed ratings are assigned to tires based on a series of test conditions. These ratings are derived from optimal conditions and near perfect temperature and safety situations. FYI, speed ratings may not apply to a repaired light truck or passenger car tire and don’t reflect the performance of a compromised tire.

From the Michelin Website:

Speed Ratings Refer to More Than Just Speed

Speed ratings make a difference not only in regards to speed, but in regards to ride comfort, wear and cornering ability. Typically, the higher the speed rating, the better the grip and stopping power, but the lower the tread life. You can always increase the speed rating of the tires on your vehicle for improved performance, but can never decrease it without reducing the vehicle top speed to that of the lower speed rating selected.

Mixing Speed Ratings

If tires of different speed ratings are mounted on a vehicle, the lower speed-rated tires should be placed on the front axle regardless of which axle is driven. This is to prevent a potential over steer condition. Vehicle handling may be affected, and the vehicle’s speed capacity is now limited to the lowest speed-rated tire. For best performance, it is recommended that the same size and type of tire be used on all four wheel positions.

From Rubber Manufacturers Association:

“Always check and follow the recommendations in the vehicle owners manual and/or the vehicle tire placard regarding the use of speed rated tires. In general, all tires on the vehicle should be the same speed rating and replacement tires should have a speed rating equal to or greater than the speed rating of the OE tires.”

“Tires with different speed ratings may vary in ride, handling and/or other performance characteristics. Thus, care should be taken when mixing tires of different ratings on the same vehicle. It is the “top speed” of the “slowest” tire on the vehicle which limits the vehicles top speed without tire failure.”

“Consult he Vehicle Owners Manual or tire placard for the correct size and speed rating of the original tires. To avoid reducing the speed capability of the vehicle, replace a speed rated tire only with another tire having at least the same speed rating.”

“If tires of the different speed ratings are mixed on the same vehicle, remember that the tires may vary in ride, handling or other performance characteristics. Thus care should be taken when mixing tires of different speed ratings on the same vehicle.”

Bottom Line:

At this point there does not appear to be any state or federal law that requires the use of vehicle manufacturers recommendations when replacing tires. However, downgrading the speed rating of your tires may affect the handling and safety of your vehicle and may also leave you and the installer legally vulnerable. Trial lawyers have discovered they can get big money in tire lawsuits. There are some tire retailers that will be willing to downgrade your recommended tire rating to make the sale. However many reputable tire dealers and installers may not be willing to do this for you for liability reasons. At Brown’s Alignment we will prefer to err on the side on caution with respect to any safety concerns related to your vehicle. We recommend that you replace your tires with the size and rating that the manufacturer concluded was the best choice for your particular car. Suggestions that tire dealers prefer to sell the higher priced tires for ulterior motives, such as to make more money are not always the case. Our profit margins at Brown’s Alignment are lower on the upper end, more competitive tires.