What are the differences between difficulty levels in Rage?

Solution 1:

The Rage wikia describes:

The enemies appear to have roughly the same amount of health on each level, but hit a lot harder and come in greater numbers as you increase the difficulty. Notable exceptions include the Large Mutants and Krakens, which seem to have lots more health, and the Authority Elite, which are basically tanks on Nightmare, requiring 3 or 4 Dynamite Bolt headshots.

Cars in the wasteland will also fight a lot more aggressively and use shields more frequently as you increase the difficulty.

Loot appears unaffected, apart from a Rocket Launcher in Kvasir's Sewer on Hard and Nightmare difficulty.