OK, dropped the pressure to 37ish and WAYYY better. Still pretty loose at 75mph, but not death defying. Thanks for the advice, never had loose tires fixed by having less air. I typically run max pressure in my Yaris and Prius.
Theory goes that at that max psi, the actual tire footprint becomes (in your case) noticeably narrower. The shoulder blocks which are on the very edge of the tire, designed for handling lateral loads, probably ended up not having great contact with the pavement. Lowering your psi meant they were able to work the way they were designed to, especially during the transition when the load goes from the centre of the tread (driving straight) onto the edge of the tire during lateral movement from turning.
Running max pressure on your Yaris and Prius serve only one pupose--to lower rolling resistance by way of smaller footprint. Your ride/comfort/NVH will be greatly improved if you were to run closer to the psi recommended on the doorsill.. probably 30-32psi.
I know that in autox, where ride quality is not prioritized, people run the max psi to give the sidewall additional stiffness to resist to rolling over, and therefore provide quicker lateral response and ultimate grip. That said, they do not run them at max psi all day--they chalk the sidewall, and lower their psi progressively, monitoring the chalk line until the sidewalls start to flex and roll over. They also reduce psi as morning bleeds to afternoon, to compensate for both increase tire pressures due to 1.5 minute runs, as well as the increase in ambient temperatures. They are also typically stripped of all unnecessary weight (ranging from just removing spare tires/loose crap in the back, to full on removal of seats)--more weight equals more demands on the tire, quicker for them to feel "greasy". Most passenger tires will run 40psi +/- 6psi, in my local climate, without running into an issue with feeling greasy on regular commuting. I run 34psi on performance summer tires, but that's for the marginal increase in sidewall stiffness, and so that my plus size (both in width and sidewall height) tires can soak up the pothole-ridden roads better. If the roads were better, I would run close to 38-40psi.
Long story short, I'm guessing that you're probably better served lowering your tire pressures on your entire fleet which is presently running on max psi. I suspect mileage decrease will be minimal, but acceleration (less psi = less weight), braking (more tire contact) and turning stability will all be noticeably improved.
