Tire Inflation Devices
Letting air out of the tires is a fairly easy process. This is known as “Airing down”.
You can learn more here.
Refilling your tires with air can be a bit harder (and more expensive). It is getting difficult to find gas stations with air compressors that actually work. And, you can drop a lot of quarters in the process.
So, there are a variety of air compressors and high pressure tanks that will do this for you.
Portable air compressors range from small and inexpensive to large and quite expensive. The smaller one will eventually air up your tires, but it may take a very long time and they tend to overheat when run continuously. Look for compressors with 100% duty cycle if you want to run the compressor continuously. The larger air compressors are great, if they fit in your budget. Some can even run air tools.
Engine driven air compressors using a York compressor are popular. They will run indefinitely and require little maintenance. You are limited by the length of your air hose(s), if you are trying to help someone else.
Portable compressed gas cylinders typically are filled with CO², and need to be refilled at a welding supply or fire extinguisher store. The main drawback with them is that when they run out of CO², you’re done. It may or may not be convenient to find a store to refill your tank, there is a $12 to $18 cost for the refill, and the tanks require a hydrostatic inspection every few years to be sure they are still safe. The same factors apply to SCUBA tanks, but in addition, you must have a SCUBA certification card to get a refill.