I call this my emergency air supply line for cars in case you have a slow leak in your car tire. What you need is two valve stem connectors to connect to a ten foot air supply line. Connect one of your valve stem connectors to the car tire that is flat and connect the other valve stem connector to the other tire that is filled up with air. This will automatically put air into the other tire equally. That way you will be able to drive your car to a garage to get it fixed.
I think we all have faced this problem at least once in our lives. You get up in the morning and get ready for work and when you go outside you see that you have a flat tire because you have a slow leak in your tire. You open your trunk and get out the emergency air supply line. You connect it to the flat tire first and then you connect it to the other tire that has air. The tire only holds a certain amount of air, maybe 30 to 40 pounds of air per tire. When you connect the connector to the other tire that has air, it will release half of the capacity of air into the flat tire. That way, you can get it to a auto supply shop to get fix-a-flat. It is very easy to get a slow leak in your tire without realizing it. This will save you a lot of time waiting on roadside assistance and help take away a lot of stress.