2010 GMC Canyon Extended Cab Owner's Manual - Page 274

Driving On Grades Reduce speed and shift to a lower gear before starting down a long or steep downgrade. If the transmission is not shifted down, the brakes might have to be used so much that they would get hot and no longer work well. Vehicles with automatic transmissions can tow in D (Drive), but you may want to shift to a lower gear selection if the transmission shifts too often (e.g., under heavy loads and/or hilly conditions). If the vehicle has a manual transmission it is better not to use the highest gear. When towing at high altitude on steep uphill grades, consider the following: Engine coolant will boil at a lower temperature than at normal altitudes. If the engine is turned off immediately after towing at high altitude on steep uphill grades, the vehicle may show signs similar to engine overheating. To avoid this, let the engine run while parked, preferably on level ground, with an automatic transmission in P (Park) or a manual transmission in N (Neutral) with the parking brake applied, for a few minutes before turning the engine off. If the overheat warning comes on, see Engine Overheating on page 6-35. Parking on Hills { WARNING: Parking the vehicle on a hill with the trailer attached can be dangerous. If something goes wrong, the rig could start to move. People can be injured, and both the vehicle and the trailer can be damaged. When possible, always park the rig on a flat surface. 5-58