central tire inflation system . I have it on my h1 hummer. Its great when you want to let the air out of your tires when going on the beach with out getting out or slowing down. Coming off the beach , just hit inflate . If I let the truck sit and have a leak in a back tire , both rear tires go flat, but just get in and crank it up and the tires fill. you always know how much air is in the tires.