Originally Posted by Dartmouth 12V

Higher rpm puts less stress on the engine because there are more revolutions of the engine to do a given amount of work. This means that each combustion cycle needs to do less work so less fuel is injected and the stress is lower. However, because you do more cycles, that affects fatigue.

The same thing is true with exhaust gas temperatures. When you downshift, you need to inject less fuel per event to get the same rear wheel torque(it isn't engine torque that matters) so you run leaner and cooler.

A lot of people do not understand the difference between rear wheel torque and engine torque. Because we have transmissions, there is no direct correlation between these two things. For a simplified example, compare an engine turning 2k rpms and outputting 400ft-lbs of torque with a 2:1 reduction to another engine turning 1k delivering 500ft-lbs direct drive. The engine turning 2k rpms is actually outputting 8/5(2*400/500) as much torque as the one outputing more torque at the crank because of the gearing even though each vehicle is running down the road at the same speed.

If you want to get the rear wheel torque, you need to take the crank torque, multiply by your transmission ratio, differential ratio, and the radius of your tires. If you do this, you will get a number in units of pounds which can be thought of as the force pushing the truck ahead. Torque is a very misleading number on its own without knowing gearing and tire size.