Originally Posted By: CapriRacer
I explain the process used to qualify tires for new vehicles here:
http://www.barrystiretech.com/oetires.html
Short version: The vehicle manufacturer decides the speed rating.
Short version, cheap is good, cheaper is better.
As a consultant to major automobile manufacturers in the US over three decades I can assure folks that a 2006 Elantra did not require H-rated tires, and that when Hyundai made the decision to use them cost was the single largest criterion in their decision. The second biggest criterion was ride.
With most passenger cars that is the situation in tire selection, with exceptions.
It is the same reason tire inflation pressures recommended are usually too low. The automobile manufacturer wants to make money selling cars, and it accomplishes that by reducing costs in manufacturing and by providing something people would like to buy. Most people want a nice ride.
There are exceptions, and these are generally special-purpose vehicles. For example, a Ferrari will come equipped with a high performance summer tire with a very high speed rating. First, the cost of the tires is a relatively small part of the cost of manufacturing. Second, the intended purpose of the car is met by tires that are quite different than what the average passenger car would use.
Every auto manufacturer considers a matrix of specifications for tread compound, sidewall construction, rolling resistance, and myriad other parameters. For example,
GM assigns a Tire Performance Criteria (TPC) number connoting each matrix.
If your personal criterion happens to match that criterion, then you should only buy exact replacement tires, same brand, same size, same everything and inflate them to precisely the pressure on the door jamb. I don’t think that’s a reasonable approach, but some people do.
Once you decide you’d like different parameters, perhaps a snow tire, or better wet performance, or better transient response, you can design your own matrix and seek tires that match it. Speed rating is just one more parameter.
Originally Posted By: CapriRacer
This leads me to the opinion that H speed rated tires should be the minimum.
That seems to indicate some confusion as to what a speed rating actually is.
Are you suggesting that an S or T rated tire could be driven at a high enough speed long enough to fail in a normally loaded Hyundai Elantra assuming proper tire inflation?
If you are not, then considering H rated tires as the minimum makes little sense.
Originally Posted By: CapriRacer
Sorry, but speed ratings are pretty much tied to the use of fabric overlays (of the belt) commonly known as cap plies.
The primary cause of failure at speed is heat. There are two ways to reduce that with the same tire pressure - decrease flexing which can be accomplished by changing cap design, tread depth, and sidewall thickness. Anyone who has worked with racing tires knows what I am talking about. Each molecule of rubber and the various compounds in the tire interact with other molecules when the tire flexes. The more molecules, the more heat. You can reduce the temperature of the tire simply by shaving the sidewalls and tread down.
This would normally result in tires which are both more sensitive to side impacts like curbs, and anyone who has owned a Ferrari or a Porsche and let his wife drive it can attest to that, and reduced mileage before wearing out. But, at increased cost, you can overcome that somewhat with design. The two tires I mentioned using - the Michelin Hydroedge and Goodyear Assurance TripleTred - use a similar “stiff” cap design to achieve a different balance of wear and speed rating than the OEM tires. Stiff cap = less flex = longer wear.
Originally Posted By: Wilhelm_D
......Interestingly the “88” tells you it is rated at 1,235 lbs per tire, arguably safer than the 87H that came with the car since it has a higher load capacity.......
Originally Posted By: CapriRacer
Not exactly. The 88 is indeed a higher capacity, but it is an artifact of the tire sizing system - in this case it is because Euro metric tires use 240 kPa (36 psi) as a rating pressure, where P metric tires use 35 psi. For practical purposes, the constructions of tire built to either sizing system is the same.
Just to be clear, the load rating is independent of the tire pressure as is the speed rating.
It is a fact that increasing tire pressures means that a given tire and wheel combination on a particular car can better perform at higher speeds or greater loads. The reason, again, is that increasing tire pressure reduces tire flex, which reduces heat.
However, the load and speed ratings are derived using a standard methodology, which means that when you compare tire A with tire B, the one with higher load rating can in fact handle a greater load, just as the one with the higher speed rating can handle the higher speed.
Originally Posted By: CapriRacer
True, but in order to pass a T speed rating the tire only needs to hold that speed for 10 minutes! That's not enough time to tell much about the long term durability.
The speed rating has nothing to do with durability. It has to do with the ability of the tire to operate at a given speed using a standard methodology.
A Hyundai Elantra could not sustain the T-rating speed for ten minutes, or one minute.
