If you want to know exactly how much air is in your tires at any time, then consider the FOBO Tire or Tire Plus. The systems’ four wireless tire sensors replace your current valve-stem caps. They attach with a special wrench that makes them theft-proof and they also use Bluetooth 4.0 to deliver tire pressure data to your Android or iOS phone. FOBO issues an alert if tire pressures drop below a preset level. If your phone is turned off or is not in the car with you, a wireless in-car monitor will also alert you to pressure issues. Purchase for $150 (replacement tire sensors cost $30) at www.my-fobo.com.