Abstract: Pressure actuated monitoring means for a vehicle having wheels carrying tubeless tires, includes pressure sensors mountable on each wheel. Each sensor has a housing receiving direct pressure of air from the interior of a tire. A flexible member in the housing moves in response to air pressure in the tire when the pressure falls below a critical predetermined magnitude. A switch on the housing has contacts which open and close in response to movements of the flexible member. The sensor is adjustable to respond to pressure of any predetermined magnitude. The monitoring means includes axles connected to the driving wheels. The axles carry slip rings continuously wiped by wiper arms carried by stationary axle housings. Alarm lamps are connected in circuit with the wiper arms. The monitoring means may also include spindles carrying freely rotatable wheels. Fixed spaced contact rings on the spindles are wiped by continuously rotating wiper arms carried by the wheels on the spindles.