One of the most common questions to our Tire Talk column is, “Why does my car have V-rated tires when I never drive over the speed limit?”
V-rated tires are capable of running at 240 km/h, speeds that would land you behind bars in Ontario. In reality, the car manufacturers know you cannot go that fast but they want you to have the maximum tire performance and grip at lower speeds.
Speed ratings on tires are really a misnomer; they should be called performance ratings.
Higher speed ratings are the result of a more robust build quality, extra reinforcing belts, and stickier rubber packed with higher-technology additive materials.
The more sophisticated the tire design, the higher the speed rating the tire can achieve. What some people forget is that, what works for grip at high speeds also works for grip at low speeds.
Kal Tire, one of Canada’s largest tire retailers, set out some simple tests to prove that a high-speed tire has more grip at lower speeds as well.
Using two Mazda 6 sedans on a test loop of everyday driving experiences, it was easy to tell the difference in performance between an S-rated (180 km/h) tire on one car and a V-rated tire on the other car.
Exercises consisted of a diminishing-radius turn (like an off ramp that suddenly gets sharper), collision avoidance (a quick lane change), slalom (steering around an object in the road quickly) and, finally, an emergency stop.
Both cars did the exercises at exactly the same entry speeds to see how well the tires worked. Distances were measured and pyrometer readings were taken after the exercises to see how well the tires enjoyed the events.
The diminishing-radius turn was taken at 25 km/h. The S-rated tire squealed in protest as the turn got tighter and the car tried to understeer. The V-rated tire did not raise any objections as the road tightened up.
Entering the rapid lane change at 50 km/h, the V-rated tire tracked to the next lane with sure-footed aplomb. The car felt stable and reacted well to the quick steering inputs. The S-rated tires howled in protest and the rear of the car started to skid.
The emergency stop was the real eye-opener. At 60 km/h, the S-rated tire took 2.15 seconds to stop and a distance of 46 feet, 10 inches (14.3 metres). The V-rated tire took just 1.13 seconds and a mere 29 feet, 5 inches (9 metres).
So the better rubber stopped an entire car-length shorter at city speeds. Extrapolate that to a speed of 100 km/h, and the V-rated tires will be stopped while the S-rated car is still going almost 25 km/h.
Afterwards, the tire temperatures were higher on the S-rated tire — 27C on the outer edge, 23C in the middle and 18C on the inner edge, compared to 25, 20 and 18 on the V-rated tire. (It was a cold day, with pavement temperature of 6C and air temperature of 8C.)
Clearly, the S-rated tire was more stressed, and stress leads to more rapid wear. Heat is the tire’s ultimate enemy. It leads to high wear rates and, in extreme cases, total failure. Over the long run, this tire would start to feather its edges and become noisy.
One interesting fact is that higher-rated tires develop more grip as they get hotter (up to a point), while lower-rated tires reach their maximum temperature much sooner and then get slipperier.
Car companies recommend tires with high speed ratings because the tire’s higher performance helps the car handle better, stop shorter, and adds a huge safety margin.
This testing seems to support Kal Tire’s slogan: “Just because your tires fit doesn’t mean they’re safe.”
“Many drivers just plug a tire size into a search engine online, and then look for the cheapest tire, ignoring speed rating and load index,” warns senior zone manager Mike Butcher. “It is much more that a top-speed issue. In daily driving, there are miscues that can happen. The correct tire can supply a better safety margin.”
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The Toronto Star for Wheels.ca