var pxPerMeter = 30; // 30 pixels = 1 meter

function createGround() {
  var bodyDef = new b2BodyDef;
  var fixDef = new b2FixtureDef;

  bodyDef.type = b2Body.b2_staticBody;
  bodyDef.position.x = 250/pxPerMeter;
  bodyDef.position.y = 370 /pxPerMeter;

  fixDef.shape = new b2PolygonShape();
  fixDef.shape.SetAsBox(250/pxPerMeter, 25/pxPerMeter);
  fixDef.restitution = 0.4;

  // create the body from the definition.
  var body = carGame.world.CreateBody(bodyDef);
  body.CreateFixture(fixDef);

  return body;
}

createGround();