Lets see if I can bring some Science and Engineering facts and history to this issue of speed limits on ST tires.
In '60's & '70 when ST type tires were "invented" and started to be
applied to Travel Trailers, the national speed limit was 55 mph and
tires were bias. Trailers were considered "big" if they were 24' long
and I doubt there were many if any 5th wheel tri axle trailers on the
Today we see speeds across the country of 70+ and there are many
locations where you could set the cruse at 70 and never slow down for an
8 hour drive here in the US. Trailers over 30' are normal with some
pushing 40 feet and most have tandem axles with more tripples showing up
The formula for determining the load capacity for all tires follows the basic format
Load = K x (air pressure) x (air volume)
Now the calculation for air volume is the complex part as aspect ratio
and a theoretical rim width and other factors such as tread depth come
into it but these details do not change the fundamental format of the
The "K" shown above is an important concept as it is really a factor based on the expected
service. Trucks are expected to carry heavy loads but not all the time.
passenger cars are not expected to be heavily loaded much of the time
and while RV are loaded almost all the time, when ST type tires were
"invented" we didn't have slide-outs or 35' 5vers or pickups capable of
running 80 mph for hours on end.
Standard passenger cars seldom if ever carry their max load. The GVWR
and GAWR are not even in every owner's manual or on the Vehicle
Certification label AKA "Tire Placard". They are expected to be run at
posted speeds but on paved roads for hours on end and driven 10,000 to
20,000 miles a year i.e. used fairly frequently with many being parked
in a garage.
For the sake of this discussion lets assume the K is set to 1.0 for passenger cars.
Now what do you do with Station Wagons and other "multi-purpose"
vehicles? These vehicles were expected to carry more load more often so
the service is obviously more severe. When SUV's came along they were
places in the "Multi-purpose category" and if a passenger type tire was
applied to a trailer that was also considered more severe service. So
the load capacity was reduced. many are aware of the "De-rating of P
type tires when used on trailers or SUVs etc. So K (multi-purpose) = K
(passenger) divided by 1.10 and we end up with lower load capacity.
About 90% of passenger.
Lets look at the actual numbers.
P235/75R15 105S 35 psi
2,028# @ 35 psi 112 mph on a Passenger vehicle
1,844# @ 35 psi 112 mph on an SUV or P/U or trailer
Moving on to Pick-up service we have LT type tires. The formula is still
K x pressure x air volume but with trucks expected to carry even more
load most of the time their K factor is different.
Their numbers give us
LT235/75R15 101/104Q LR-C
1,985# single 50 psi 99 mph
This lower load capacity on truck service is clearly because the higher percentage time spent carrying more load.
Before we move on lets look at the ST numbers
ST 235/75R15 LR-C
2340# @ 50 psi 65 mph
To me the obvious question should be: How does the addition of the
letters "ST" on the sidewall allow a 26% increase in load capacity over a
P type tire (adjusted for trailer service)
or a 29% increase over the heavily loaded but occasionally empty truck?
The only reason I can see is the significant reduction in speed.
We all know, or should know that more load (more deflection or bending)
generates more heat so what could you do to counteract the increase in
heat due to the increase in load? Obviously lower the speed would reduce
the higher heat and that was part of the original ST tire standard.
Now lets look at the tire type that is of real interest. ST type as used on many RV trailers.
In 2014 new duties were imposed on imported tires but ST type were
exempt, sort of. There were various requirements some of which were
requested to be changed or eliminated. The speed symbol was one of these
Starting in 2017 (possibly earlier in small quantities) many ST type
tires started showing up with a Speed Symbol selected from the table as
published by US Tire & Rim Association in the LT section.
The problem is that Speed Symbol does not have any standard DOT test or
requirements as in the US Speed Rating is really a marketing tool and
not a strict performance requirement. A review of various ST tires shows
a range of speed symbols from L (75 mph) to R (106 mph) and possibly
Further compounding the confusion is that the speed symbols are from the
SAE - Society of Automotive Engineers and according to SAE their test
apply to ""standard load," "extra load," and "T-type high-pressure
temporary-use spare" passenger tires." This raises the obvious question
of what test procedure, if any, are various tire companies following
when they assign the Speed Symbol? While we are talking about SAE
symbols we need to remember that DOT does not recognize or test for
Let me close with a question I have asked a number of times but as of now have never received an answer for.
What "magic" pixie dust are tire companies putting in their ST tires
that allows them to run 75 or 81 or even 106 mph without making any
adjustments in load or inflation? and If they have this "magic"
engineering available, why aren't they using it in their LT tires?
may want to argue that tire technology has improved since 1970 and that
is certainly true but I would ask why haven't load capacities for
Passenger or LT or heavy truck tires been increased over the past 50