Thats the metric for a 31x10.5 tire, not all 31s. 235/85-15 is a 31, as is 245/75. They are just narrower.
I would never ever put passenger car tires on. Always light truck. They last so much longer and are more durable.
Just referring to more of it being a close to comparable size in terms of height and width within about 1/2" or so when put against the typical option when referring to 31s which is the 31x10.5. I know other slight variations stay close to size, and breaking down every tire to just the basis of a true 31" diameter in general would have a lot of unnecessary running around, and that is just the variation I happen to run, which I will admit that I was a little more concrete in the way I said it, which I am sorry for having failed in that way.
I'm running Cooper AT3s, and have been fine in terms of wear. I know the LT list as a stiffer sidewall, but the passenger for these at least (not saying it holds true to all tires, but simply one example of a popular tire) actually lists higher load rating while the LT has the higher load range (more stating just for it being interesting, not to argue any point), and the passenger tires tend also carry longer tread warranties at a slightly lower price ($10 less for an extra 5,000 miles at Discount Tire price) as well.
For the everyday driver, I would stand behind the passenger tires in that size or smaller, but if you deal with more load and stress, then I would say go with the LT tires to cover that added work on the sidewalls. These are just 1/2 ton trucks, so they have their limits and tires are a little lower on the list, as long as you have a decent set, until you build up the rest to take more than they were intended, but I would agree that if I stepped over a 31 equivalent, I would go the LT route instead.
Bottom line I think we can all agree on, 235/75r15s just don't look right on our rigs