I really feel for the OP, and it hurts me to see this kind of behavior. When I was in college, I was the person without a car. It stunk. Fortunately I had two friends with cars who were nice enough to take me places. I always felt bad asking them for rides, so I made myself a few rules:
1) 90% of my rides were to get groceries, so I'd usually make one large trip to last me a month or two. I planned ahead, so I wouldn't be calling them up last minute, or so frequently that it became a burden to the person with the car. I also tended to switch off between my two friends to spread the driving even more.
2) I was on their schedule, not mine. They had the car, and I always kept the fact they were doing me a favor firmly in my mind.
3) If the errand was for me(groceries, a trip to the craft store, etc)I paid for gas, usually by giving them $5-$10. If it was more of a fun time outing(a movie we both wanted to see, etc), I'd usually pay some portion of the expense. Sometimes I'd cover their ticket, sometimes I'd cover the snack expenses, that kind of thing.
4) I usually paid for a stop at a local fast food place on the way there or on the way home, just as an extra thank you for taking the time to drag my car-less butt around town. We didn't eat at the Ritz or anything, but you'd be amazed at how happy college students are to drive someone around if the words "free dinner" are used:p $5 for a combo at Arby's was a powerful motivator. And of course the bonus was that I also got to eat at places that were usually off limits to me due to distance!
I did tend to be over generous when it came to my friends driving me around. I did this for a couple of reasons. First, I always felt so bad asking them to take me places. I *really* didn't want to become that friend who only calls when they need something. And second, I also didn't want my friend's good graces to dry up on me. *All* the major super markets in the area were outside of walking distance, so unless I wanted to be eating overpriced expired food from the local liquor store, I needed access to a car! I was doing this for a good 4 years which would have become very unpleasant, very quickly without some car access from somebody.
Having been on the receiving end of the kindness of friends with cars, I can say without a doubt that you're being taken advantage of, and the people who are asking you for rides have long since forgotten that what you're doing for them is a favor, not a requirement. No, you're not being petty telling your roommate that you can't take off of work to drive her home. Who does that? It's not petty to ask for gas money either. You're paying A LOT more than just gas money for your car. Even if you don't have a car payment, you've got gas, insurance, registration, maintenance, and repairs to deal with. Oil changes and new tires aren't free! If they were paying for every drop of gas that went into your car, even for trips that they weren't involved in, they'd still be getting the bargain of a lifetime.
If they can't be bothered to treat you with respect and show the bare minimum of gratitude, maybe you shouldn't be bothered to drive them around. The really sad thing is that you're probably going to end up being the "bad guy" if you start telling them you can't drive them around anymore, at least for a while. If it was me, I'd be sorely tempted to get a copy of the automotive section of the local newspaper to give them next time they try to take advantage of you, or grumble about you being unwilling to drive them everywhere at a moment's notice.