A guy at the tire shop determined that the reason that tire loses air is that there is a rough area (likely damage from one of the earlier flat episodes) on the tire bead. He put 80 psi in it and there was no leak when submerged. But these tires are 6 ply and only rated at 45 psi so we dropped it to 45. Although the L5740 manual specifies only 20 psi for R4 on the front, I've been running at close to 30. One day after getting home that tire was at 38psi. My shop floor is probably colder than the tire shop right now, so that may be some of the loss. I inflated back to 45 just to see what happens. Will check it in a couple of days. Meanwhile, just to clarify, my R4 fronts don't look great close up. There are chunks missing due to running tire chains or from sharp rocks, and the edges are so round that they're even worse on slick surfaces than ever. That's why I can see spending money. No intention of replacing the rears until they need it. The difference of R4 and R14 for the front is about $50 per tire. Not that big of a deal.