RE Speed Ratings and operation speed.
My comparison to engine
redline seems to me to be the easiest for many to understand. It is
possible to run an engine right at redline or even above for a short
time but I can't find anyone that advocates operating an engine at the
rated max speed for any significant time or distance.
test is not a DOT regulatory test but is a test based on Society of
Automotive Engineers testing. To be marked with a speed symbol a new
tire needs to run for 30 minutes on a smooth drum at the stated speed.
is no requirement for a tire to be "conditioned" with a few thousand
miles at lower speed over potholes and up on curbs or with 110% of the
rated load or for the tire to be able to pass DOT regulatory durability
tests after running the SAE High Speed test.
increasing number of ST type tires now come with a speed symbol molded
on the sidewall. The primary reason for this seems to be to avoid import
duties. What many want to ignore is the fundamental truth that the load
capacity of ST tires is much higher than an LT type tire based on the
premise from 1970 that the ST tire would be on a single axle trailer
that was limited to 50 mph operation speed.
Molding the letters "ST" on a tire is not magic. Physics still applies. If people want to
drive at 75 or 80 while towing as they would with their LT type tires
and not have failures, then I suggest they pay attention to the Physics
and limit the actual tire load as if it were an LT type. This is easy
First simply look for an LT type tire with the same
dimensions i.e. ST235/75R15 > LT235/75R15 in the Load tables and
limit their measured load to the number found in the LT tables.
you do that you will probably see a significant reduction in tire
failures. Of course this also means you are not in the 50+% of RV owners
that operate your tires under-inflated.
If you ignore the Facts
and Physics of reality you will have to live with the consequences no
matter how much you want to believe otherwise.
Bottom Line. If you want durability and life more like LY type tires then treat them as if they were LT type tires.
IMPORTANT NOTE and WARNING
As with the engine in your RV or tow vehicle it may be possible to run faster but it does not mean you will avoid all problems. This post is just about tires and not about safe and reasonable operation of your RV. Personally I think 70 is too fast to drive a "big rig" and 75 is certainly too fast for towing. I have heard comments about an increase in truck tire failures due to increased speed limits in many states. A MAX of 75 is stated in some tire information guides published by major tire companies and as with any maximum, the closer you are to it the more likely you will have some negative consequences.
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