Long, steep hills can challenge a truck unless the gear selection lets the engine run at its most efficient RPM. Vehicles develop their peak horsepower at what used to be considered fairly high revolutions. The 1995 Jeep Wrangler for example is rated at 100 rear-wheel horsepower at 4,500 RPM and maximum rear-wheel torque of 131 at 3,000 RPM. The 1995 Ford Ranger is 90 hp at 4,500 and 140 torque at 3,000. Most current vehicles have similar specifications. That means to get the maximum horsepower, or maximum torque you must select a gear and range that will pull the hill with the engine turning at the best RPM. High revolutions may sound as if the engine is about to take off, but they are far better than an engine that is lugging and will be much kinder to the engine. Failure to make the right selection can result in overheating.
As an example: on the hill to Belfort the trucks which arrived with no problem used low range low or second when the hill got steep and the pull long.
For downhill you need to select a range and gear that will keep the appropriate safe speed with only occasional use of the brakes.