I tested mine today, since my tires were a bit low. I filled them both, then used a high-quality Slime analog gauge to set both to exactly 42. Ambient temp was 71 degrees (unseasonably cool and beautiful day). Later, I took the bike out (72 degrees). Initially the TPS showed 42/42 and after several miles, it said 43/42. Several hours later, at night, at 62 degrees, they consistently said 42/42.
So, in my case, they are pretty darn accurate.