Some clarifications and comments on statements in this thread from a Tire Engineer.
"An "LT" designation on a trailer tire size specifies load range only. It is not designed for use on light trucks.
Actually the first letters of the tire size indicated the "Type" tire as specified by Tire & Rim Association
. P would be for Passenger vehicle service, LT is for Light Truck and ST is Special Trailer. The load formula that generate the numbers in the tables are slightly different for each "Type" tire and is based on the expected service conditions and usage. There is no such designation as an LT tire for trailer application.
"Load Range" is a letter code C, D, E etc that identifies the max inflation for that specific tire. Load Range is molded on the tire sidewall and is at the beginning of the tire size marking.
While vehicle manufacturers can select P tires for a Light Truck application there are adjustments that are required for the load capacity. LT tires may be selected by the manufacturer for use on trailers but the load capacity of the LT type tire must be used not the load capacity for a similar size ST tire.
ST tires are never to be applied on a vehicle intended to carry passengers.
"Always inflate trailer tires to the maximum inflation indicated on the sidewall
This is correct for trailer application but not always correct for motorized vehicles such as cars, light trucks or motorhomes.
"The combined capacity of all of the tires should exceed the loaded trailer weight by 20 percent.
This would be a good guide and I would support this but it is not a requirement
RE Actual weight: Since it is documented that a majority of RVs have one or more tires overloaded, you really need to get your unit on a scale when fully loaded at its heaviest and confirm you are not exceeding the tire or axle or RV max rated capacity. Here
is a web site that tells you how to weigh and calculate to learn the actual tire loads.
"In approximately three years, roughly one-third of the tire's strength is gone
." I have never seen data that supports this statement. I am also not sure what is meant by "strength" as the tire is designed to contain the air pressure so that would be it's "strength". It is the air pressure that carries the load, not the tire.
"Three to five years is the projected life of a normal trailer tire. It is suggested that trailer tires be replaced after three to four years of service regardless of tread depth or tire appearance.
I agree with these general statements for trailer application but this is based on the fact that trailers place significantly more internal stress on the tire structure due to a number of factors.
1. Tandem axle place high side loading forces on the tire structure as soon as you start turning.
2. Most trailer manufacturers select tires with very little or no margin of safety for load capacity when compared to actual usage
3. Almost all ST type tires are operated for some amount of time at or above their "red line" speed limit of 65mph. This generates excess heat which consumes the tire life at an accelerated rate.
Statements on the ST tire polyester and steel being larger or stronger may or may not be true.The rubber also may or may not contain more weather resistant chemicals. I am not aware of any broad study that would support these claims.
Hope this clears up a few things.