Can you really tow your trailer at almost 90 mph?

 In September 2024 I wrote about the operational limits on ST-type tires. I suggested that the published 65 mph speed limit was a realistic compromise. But apparently many people think they can get tires with some “magic” rubber in them so they can safely drive faster. Therefore, I am again writing about the “speed rating” of ST-type tires.

 

In the U.S., “speed rating” is really a measure of tolerance for high heat and is based on a test with speed increasing every 10 minutes until the tire fails. In Europe, Asia, and the rest of the world, they do not offer an ST-type tire because their speed tests are more realistic and stringent.

When radial ST-type tires were introduced to the U.S. market some 45 years ago, they were given a higher load capacity rating than a same-size LT tire. However, the trade-off was stated in the industry standards books as having a max operating speed of 65 mph, with load and inflation adjustments required if operating above 65 mph. With 50 years of tire design and testing under my belt, I know of no “magic” rubber that will deliver greater load capacity for a given size and inflation without some trade-off.

Five variables for operation speed

There are only five variables that control operation speed: size, load, inflation, speed and the ability to be rated for operation with passengers in the vehicle. So, unless you invent some “magic” rubber, you are limited by physics.

Yes, materials have improved over the last 45 years after the switch to radial construction, and we have seen improvements in tire performance. But have you ever asked why a company can make a tire of the same physical size and same load capacity at a given inflation and somehow make one tire carry significantly more load than the other without some trade-off in operation speed?

 If I had invented some “Super Magic Rubber” for ST-type tires that gave them “Super Strength,” why wouldn’t I put the Super Rubber in the other tires I make and sell and take over the worldwide tire market? Any company can ignore reality and make a marketing decision and claim a “Speed Rating”.