About vehicles kills in Borderlands and experience/items gain

You do get way less experience for killing things with the vehicle vs. killing them while on foot. I'm honestly not sure about the quality of drops being different. But in my experience playing, if I cared about xp at all, I always stepped out of my car. I don't remember really specific numbers off the top of my head, but for example, killing a Badass Spiderant King with the vehicle awareded me half (less even, I believe) the xp versus stepping out and shooting it. This is not a bug and is working as intended, so you can't just cheese your way through the game.

Later on in the game, enemies will tend to do too much damage to your vehicle too quickly for it to be useful for seriously clearing out areas. If you're just going from point A to point B, sure, you can often run over whatever you want on the way (keeping an eye on vehicle health of course). I would park it outside of a hideout though if you're going in, as enemies will chew through it fairly quickly later on and it'll explode, leaving you pretty damaged, if not already on your knees needing a Second Wind.

Also, when you start your second playthrough, the enemies damage the vehicle way more when you run into them and will sometimes explode after hitting one enemy!

Not only do you get less XP, but you're also missing out on increasing weapon proficiency.

Essentially, vehicles are good for getting your numerical achievements (kill 2500 Skags, etc), while on-foot kills are good for experience.

A short time into the game, your vehicle gets one-hit kills on skags and spiderants, allowing you to rack up your numbers as quickly as possible - but this method doesn't offer much in the way of experience.

Some of the best (re-visitable) places to rack up experience on-foot include Old Haven, The Crimson Fastness and The Descent.