I recently learned of this ingenious invention. I will eventually have to replace my tires, but I don't have the money, so for now, I try to stay on top of making sure the tire pressure is accurate. Although it doesn't take long, stooping down to check my tire pressure on a regular basis hurts my back, so I decided to invest in a set of tire monitor valve caps. What they do is change color, depending on the air pressure in your tires. I put them on yesterday, and thankfully, 3 of my tires are fine, but the one that always seems to lose air pressure more quickly shows a slight deficiency, so I'll take care of that the next time I go out. These devices vary widely in price and also level of function. The ones I got were very low cost, and I purchased them on eBay, from China. I was a little unsure of whether they'd actually work, but I'm happy to say that I definitely got my money's worth, and then some. I have a friend who is watching a set that costs around $100, but if I had that kind of money, I'd set it aside to use for the purchase of tires, and buy the cheaper tire monitor valve caps instead. The $100 set tells the actual tire pressure, rather than just changing colors, but I don't need that level of information.