While driving the truck, you take fall damage only if you land on a wheel, landing upside down or on the nose causes no damage. To add to this, when driving a vehicle and passing through gas with a gas mask equipped, you take gas damage, this seems simply unimplemented though. Another odd piece of behaviour with trucks again related to wheels is that it seems the chassis is a unmovable rock in terms of collision whereas the wheels have a tendency to send it flying at the slightest provocation even the engineer can move them(this is really a feature in disguise, its fun to push your beached truck out of the water with some struggling)