A system for supply of LPG/methane, ammonia, or gas for supply of petrol or diesel engines, which can be connectable to the electronic control unit (8) provided on board the vehicle, the system being equipped with a gas tank (1); a valve (2), a line (3) for delivery of the gas, a rail (6) for supply of the gas to the engine, a pressure and temperature sensor (7); injectors for the petrol (10), and injectors for the gas (11), is characterized in that, to carry out a continuous variation of the pressure of the gas supplied to the injectors (11), it further comprises an electronic pressure regulator and heat exchanger (5) with injector (4), as well as a control unit for deviation and regulation of the pressure (9), which, during operation of the engine with LPG/methane or ammonia or gas, is designed to receive from the electronic control unit of the engine (8) electrical signals as a function of the injection times, the engine r.p.m., and the signal sent by the lambda probe and/or by the rheostat of the accelerator/said electronic control unit for control and regulation of the pressure (9) being further designed to process said signals and send them, at a variable frequency, to the injector (4) of the pressure regulator and heat exchanger (5), which, in this way, is designed to vary continuously and with a constant progression regulation of the pressure of the fuel with which said gas injectors (11) are supplied.