Edit in JSFiddle

var game = new Phaser.Game(800, 600, Phaser.CANVAS, 'racing_game', { preload: preload, create: create, update: update });

function preload() {
    game.load.image('circuito','http://imgur.com/omJhWl3.png');
    game.load.image('coche','http://imgur.com/T7OdhYY.png');
}

var cursors;
var velocidad = 0;

function create() {
    var circuito = game.add.sprite(0,0,'circuito');
    coche = game.add.sprite(300,100,'coche');
    cursors = game.input.keyboard.createCursorKeys();
    game.physics.startSystem(Phaser.Physics.P2JS);
		game.physics.p2.enable(coche);
		coche.body.angle = 90;
}

function update()
{
                if (cursors.up.isDown && velocidad <= 400) {
                        velocidad+=7;
                }
                else {
                    if (velocidad >= 7)
                        velocidad -= 7;
                }
                        
                coche.body.velocity.x = velocidad * Math.cos((coche.angle-90)*0.01745);
                coche.body.velocity.y = velocidad * Math.sin((coche.angle-90)*0.01745);

                if (cursors.left.isDown)
                    coche.body.angularVelocity = -5*(velocidad/1000);
                else if (cursors.right.isDown)
                    coche.body.angularVelocity = 5*(velocidad/1000);
                else
                    coche.body.angularVelocity = 0;
}