Found 235/75R15 in 6pr/ C-load with loadindex 104Q singleload is maxload 900kg/1954 lbs AT 55psi ( EUR) or 50 psi ( US) , and that might be one part of the tirefailure.
If 55 psi and calculated back to 50 psi still 101LI/ 825 kg/ 1819 lbs loadcapacity, so x 2 is 3638 lbs axleloadcapacity is a good 20% reserve to GAWR's, so would not suspect to give tirefailure by overheating at 50 psi cold.
But these lighter TT's are sooner overloaded, and unequall weightdivision over the 4 tires possible, wich would need higher pressure.
Only way to check that is weighing per axle or axle-end in the loading you use it.
If overloaded ( forbidden but does not mean it does not happen) and you cant solve it, a higher pressure would be needed.
Then tires wont fail with that higher pressure by overheating, but axles can fail by overloading, also not wanted.
But if you did not experiënce any tirefailure, its not needed to switch to D-load/ 8 pr( plyrated) .
Your tires now have comfortable reserve to GAWR's and GVWR.
Look on sidewall , to check the specifications. Mayby already D-load in C-tyre. Then you mistaked the C behind as loadrange C, while it was the C for comercial ( LT equivalent) , wich always placed behind the sizes, and not as LT in front of the sizes.
You need to collect next.
1. Maxload or loadindex
2. Loadrange or plyrating, but better search for referencepressure ( behind AT).
3. speedcode, if Q or above maxload is given for upto 160 kmph/ 99 mph. ST is a more complicated story, can be for this size for N speedrated calculated so 87 mph, wich gives a higher maxload.
Then still I derate them to Q or higher, to give a deflection that wont overheat, driving the rediculesly high speed of 99 mph, so max reserve at wich no screws tremble loose or rivets popp.
And C behind the sizes could also be the notation of a C- tyre ( C for comercial, eur equivalent of LT) and then it can still be a D-load/ 6 pr tire, with its higher maxload AT 65 psi. This you all have to check.
