[ShrimpBurrito]

I was in your exact same situation several years ago. First had load range C Marathons that blew, so I switched to the same load range D Kumhos you have. After about 4 years, I had at least one blowout with them as well, which I attributed to age as the aggravating factor in combination with high ambient temperature and being too close to that tire's weight capacity. The Kumhos seem to run fine for 3-4 years, after which they are done.

As a result, I went to a 15" Carlise tire, load rating E, choosing that tire because that was the only thing available at the shop I had to visit after my blowout on a late Saturday afternoon in nowhere's ville Colorado. After 4-5 years of trouble free service with those tires running at 65 psi, I preemptively swapped them out for new tires, this time choosing the Maxxis 8008's, again, load range E. Have had them now about 2 years, again running 65 psi, and continue to have trouble free service.

Last blowout was the Kumho.

My conclusion after all of this is that tires rated for load range D, and certainly load range C, has a weight capacity that is too close to the weight of the trailer, thus leaving little safety margin. Granted, the extra capacity of a load range E tire only comes into play when they are inflated to their max pressure of 80 psi, the additional plies of this heavier rated tire seem to be enough to ward off the blowout gods. I ran them for a day or two at 80 psi after first getting them, but it shook the trailer to death, and so after some experimenting, I ended up leveling off at 65 psi.

So for me, a 15" load range E tire inflated to 65 psi has been the magic formula, and we travel on some really crappy roads in ambient air temps ranging from 30 to 110 degrees.

I think using a 14" spare would be fine. You're only talking about the spare side being 1/2" lower, so you probably wouldn't even notice. Compare that to the several inches of difference often seen with a doughnut in smaller vehicles. But I would still swap it out for a 15". Why? Imagine you hit a nail, or otherwise have some tire failure, and you're in BFE Nebraska on Sunday afternoon. You still want to drive another 6 hours. Are you going to want to risk stranding yourself on the road, or sustain significant damage to your TM, given that your 14" tires have a demonstrated propensity to fail? It's a judgment call obviously, but I consider the $200 to buy a new 15" wheel and tire cheap insurance.

Dave