When backing a tractor under a trailer, what is the recommended gear to use?

When backing a tractor under a trailer, using the lowest reverse gear is recommended because it provides the most control over the vehicle. This lower gear allows the driver to maneuver the tractor slowly, making it easier to align the fifth wheel with the trailer's kingpin accurately. By engaging in a low reverse gear, the risk of losing control or causing damage is minimized, as the slower speed enables better response to any necessary adjustments while positioning the tractor. Additionally, it allows for a smoother approach to the trailer, reducing the likelihood of sudden jerks or movements that could complicate the coupling process.