Tire load index rating of a tire is a numerical assigned value used to compare relative load carrying capabilities, the number varies with psi and it’s an industry standard so All manufacturers have to adhere more or less to same standards for weight handling.
In principle you want tires to be able to support your GVWR. So what’s a 4th gen Ram 1500 fully loaded GVWR? 6900lbs. GAWR at each axel is 3900lbs.
275/60/20 LT at 39psi tire is technically able to support 2072 lbs x 4 =8288 lbs. 1388lbs more than your GVWR and 244lbs than your GAWR. curb weight in this trucks is what 5500lbs for a full optioned crew cab 1500? And a crew cab 1500 is what 3241 lbs front with the Hemi, 2261lbs rear if you want to be more technical ? Your tires only need to be able to support 1620 lbs each in the front at a minimum, up to 3900lbs in the back fully loaded. Sure you'll have people exceeding the limits, and you have those who will never carry much weight.
A 275/70/20 at 35 psi is rated at 1920lbs carrying capacity. You do the math.
BFG wants to play by the books and will recommend at least 54-55 to match the 39psi load index rating of the same tire in p rated index at 2601lbs per FCA specs. When you call them the guy/lady is prob reading from their lawyers approved responses doesn’t mean is wrong though, Problem at 55 is that you’ll have a harsh ride if you don’t have a heavy loaded vehicle.
At 45psi for example that LT tire is able to support 2290lbs per corner or 9160lbs total.
Say you went bigger with a 20” tire say 35x12.5x20 LT, they'll say you want 50 psi to keep the factory load rating per the industry standard using the same 2600lbs per corner At 39psi FCA recommended. At 39 psi you’ll have 2267lbs support in each corner with the 35x12.5x20 LT tire. At 50 psi that will be 2680lbs, but she’ll ride harsh. In either case you’re exceeding your GVWR capacity.
One is matching factory specs, none will kill your truck. The same size at 35psi is able to support 2095lbs, same size at 25psi is rated at 1655lbs. Do the math.
Not all tires in LT will need this amount of psi, take a 37x12.5x17, you get 2150lbs at just 25psi, 2755lbs at 35psi.
Take a 315/70/17 LT tire 2535lbs at 35psi, 2655lbs at 39psi, 2915lbs at 45psi, again do the math.
You need to look at the placard to know your factory max load weight capacities. Manufacturer also included psi for those stressing about a number if you don’t want to hand calculate on your own and they have determined with that much psi you get max mpg and tire life with whatever tires they have included as factory options.
It doesn’t mean if you run less than 39 you’ll crash the moment your psi goes 1psi less than that. In stock trim your low pressure light comes at what 32psi, and I’d think they have to guess you’re loaded as to not exceed 6900lbs at least right?.... it doesn’t mean an LT tire at 39psi in the same size will be unsafe or not been able to carry max load because BFG told you to run 55 psi in this application. They’re matching factory load index specs. But you can do the math one more time and you'll know how many psi you need, is not rocket science.
My 10 Silverado 1500 crew cab 4x2 was rated from the factory at 6800lbs GVWR, 3650 GAWR front, 3950 GAWR rear, factory tires were P245/40/17 at 35psi. When you refer to the index table for a tire that size p rated that’s 2205 lbs per corner. 100lbs less advertised max mass compared to the RAM, yet can carry 50lbs more weight than my Ram in the back and still they gave us tires rated at almost 400lbs less per corner compared to what FCA said I should run in my RAM. I was running 35psi in that one with 285/70/17 E rated tires chalk tested, 2105lbs per corner, they exceeding my GVWR. I’m not running 35psi because the placard says so. you know your weight limits, you know the vehicle manufacturer recommendation as far as psi and you can use any psi combination along those lines as long as your choice supports your vehicle full gross vehicle weight rate, from there you can adjust for tire longevity, mpg and anything else that gives you happiness.
