Python for increasing speed of vehicle (gears)

Hi i’m working on a car level…that involves the car increasing it’s speed to 3 levels

this simple code will make the car move at increasing speed at 2 levels

    move = cont.actuators ["move"]    
    mouse = cont.sensors ["mouse"]
    stop = cont.sensors ["left"]
    if mouse.positive and own.position.x > 0.1:
        move.dLoc = [0.6,0,0]
    elif mouse.positive:
        move.dLoc = [0.1,0,0]
    if stop.positive:
        move.dLoc = [0,0,0]

how do u make it so that the car could increase to a third speed level?

#also… how do i make the car to stop moving when the mouse is not pressed