Before entering a curve.

Slow down to a safe speed, and downshift to the right gear before entering the curve.

This lets you use some power through the curve to help the vehicle be more stable while turning. It also lets you speed up as soon as you are out of the curve.

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Special conditions where you should downshift are:

Multi-Speed Rear Axles and Auxiliary Transmissions

Automatic Transmissions

Retarders