Review on Enhance Vehicle Maintenance: 20 Pack Red Tyre Valve Dust Caps for Car, Motorbike, Trucks, Bike, Bicycle by Julio Avikunthak

They are pretty much identical to those in your car. If you inflate your tires regularly at the gas station (and you should!), you'll end up losing the caps. It lets dirt and air through. Just keep them in the glove box and don't worry. They fit well and look like the original caps. And they're cheap. You can't beat this.