The valve stem caps that come with your tires stink. They are hard to get on, easy to fumble once you've got it off, and the wheel covers rarely give you enough clearance to get your fingers on them in the first place. This cap extends beyond the wheel cover and gives you a nice nubby surface for twisting. It also has an unthreaded lead-in section that allows you get the thread started much more easily. It is for standard automotive tire stems with a 0.302" (7.7mm) diameter thread. When you first screw on the cap, the stem will form a mating thread in the cap. I've made the section of the cap for the thread interrupted so it easier to form the thread.

The unthreaded section of valve stems, however, can vary in diameter depending on whether there is rubber covering the stem or not. I've included 3 different versions that will accommodate 8mm dia. stems (no rubber overmold), and also 9mm and 10mm diameter stems.

I printed mine in PLA, but PETG or ASA would probably be a better choice.