By Roger Marble
I saw this post on an RV Advice forum: “We have ST-type tires. Do we have to run less than 65 mph? We see a lot of RVs passing us.”
The “speed symbol” is actually a relative measure of heat resistance.
The speed number is based on a brand-new tire being able to run for 30 minutes on a perfectly smooth steel drum at a constant speed without coming apart. Any tire running the test is considered scrap after the 30-minute run.
65 mph is the assumed Max operational speed for ST-type tires.
This is part of the calculations used to establish a tire load capacity. While it is unlikely that your tires will come apart as soon as you run 66 mph, it is true that the faster you run, the “faster” you are using up the life of your tires. People running faster than 65 is one of the contributing reasons for ST-type tires to have shorter life than tires on your car or pickup.
I often compare tire speed rating like your engine “Red Line” speed. If your car indicated 4,500 RPM as the Red Line, you can probably run 4,495 RPM for some time without the engine blowing up. BUT I would think you would know this will significantly shorten the life of the engine. It’s the same with tires.
Have a tire question? Sign up for Roger Marble’s new Facebook Group: RV tire news, information and discussion, hosted by RVtravel.com and moderated by Roger. He’ll be happy to help you.