How many times have you heard this question: “Why do I have to pay extra for a Z-rated tire when I never drive over 70?” (We’ll ignore the fact that the questioner ran in a 90 mph pack on his recent trip from Memphis to Knoxville. The late British journalist L.J.K. Setright called the highway that connects the two – Interstate 40 – the fastest autobahn in the world.)
Now, here’s a question you probably haven’t heard: “Why does my Lexus stutter at 150?”
It was my mother on the phone. Despite her genteel Southern upbringing, she tended to get right to the point on phone calls.
“Good morning to you, too, Mom,” I said. “What you experienced was the electronic speed limiter.” (Notice I was not surprised that Mom had bounced against the limiter.)
“Why do they have that?” she asked.
Rather than discuss the agreement among most auto companies to limit their fastest cars to 155 mph, I just said: “Toyota doesn’t want you going any faster than that.”
“Why?”
“If your tires are underinflated or damaged, they might fail.”
“Can the thingie be disarmed?”
Forgive me for lying to my mother, but I said: “No.”
“By the way,” I asked. “Where were you going 150?”
“Alabama.” No need for further questions: That told me she was running a buck fifty because she was late for church.
Fast forward a few years and my Mom had handed down the LS400 to my brother. He needed new tires. Of course he asked the tire salesman why he needed a Z-rated tire when the same size from the same manufacturer with the same model name was available as a less-expensive V-rated tire.
Of course, I got another phone call.
The problem: I didn’t know – couldn’t know – the full answer.
My short, incomplete answer was the same one I give any time somebody is replacing tires: “Between the tire and car companies, nearly a million dollars was spent developing the tires that are on your car. If you like how the car rides, handles, how quiet it is, and how your tires handled deep water when they were new, replace them with the identical tire. And not just the same manufacturer and model, but the same part number. Why would you take a chance on messing up what you like for $200 (or less)? If there’s a performance category you don’t like (other than price or tread life), try an alternative.”
Another short, incomplete answer is that while you may never exceed 80 mph, perhaps your son, brother-in-law, or the person to whom you sell the car may. Or maybe Mom will borrow it when she’s late for church. If a lower speed-rated tire fails in those situations, you will be heart-broken at best and could lose a huge lawsuit at worst. (And that fact also would ring true for the dealer who sold them.)
Cars today can reach speeds that were unheard of in the 1970s. Back then, Car & Driver did an article of all the American-made vehicles that could “double the double nickel.” There were 10 U.S.-made vehicles capable of going twice as fast as the hated 55-mph speed limit. One of the vehicles was a Dodge pickup that was exempt from the then-new emissions controls because of its “gross vehicular weight rating.”
Today, a Honda Civic would easily blow past 130 mph if not for its electronic speed limiter. This is to say, trying to save a few dollars by installing a lower speed-rated tire does not make sense.
Digging Deeper
A more complete, but possibly inaccurate, answer is that there may be absolutely no difference between the Z- and V-rated tire except the labeling on the sidewall.
It’s not unusual for tire companies to put two labels on the output of one tire mold. If they work their dimensions just right, a single mold can produce two different size tires. (See: “Tire Size Allowances: All 242/43R17s Are Not Created Alike,” June 2011)
In the same manner, it’s not unprecedented that a V and a Z come from the same mold. However, your chance of learning that – even by accident – is pretty much zero. And I wouldn’t be shocked if I got a nastygram from a tire company for revealing that.
Another factor is the lawsuits that plague the tire industry. When a tire company can lose a suit over a tire that was purchased used, improperly repaired at least twice, and run underinflated and overloaded for thousands of miles, the industry develops a healthy level of fear.
Plaintiffs’ attorneys have kicked more than one tire company for failing to use a nylon cap, one of the features that allows the tire to pass the speed-rating test, even when the feature wasn’t required for the intended use of the tire. Also, it’s become easier and cheaper to build tires with nylon caps. Since nylon caps provide other benefits, tire companies are beginning to liberally employ them.
Much of the speed rating issue is the tire companies’ own fault. Back in the day, they planted in the mind of consumers the fact that higher speed rating meant improved traction. But the two are only peripherally related.
Yes, most sticky tires have high speed ratings, but it’s not the speed rating that provides that grip. Try this analogy: Most bankers wear suits and ties, but wearing a suit and tie does not make one a banker. The stiff belt package and cap required to meet the speed rating helps handling, but it’s the rubber compound that provides the grip.
The next time an old lady tries to get you to install a lower speed-rated tire on her car so that she might save a few dollars, ask if she’s ever late for church.
