The left motor is geared down 3 times (8:24) to steer the front wheels
The drive motor is placed on the right side and is not geared down.
The (black) steer boom is moved to the front to make more space for the gears. I added extra side beams for more stability.
Unfortunately I had to remove the engine, so I could place the lightsensor.
See also the Ldraw file for more detail on the gears.