Give the OP a break. She's clearly having a difficult time both with the situation she finds herself in and making herself understood. I fear t threads heading in the worst possible outcome route, and by that I mean if the OP thought that since nobody is willing to listen or understand, many, me included, are making unreasonable suggestions, and so many are laughing at her and her predicament that asking for help in the future is pointless, and she continues to be taken advantage of by those willing to exploit others.
Not everyone has the confidence to be able to start a pointedly difficult conversation where the other party is almost certainly going to get a little upset, probably quite defensive of their actions, and/or aggressive/passive-aggressive towards the conversation starter, and which has a significant chance of turning into a very uncomfortable confrontation. Not just that, but given that she's living with her parents, the OP is possibly still quite young, especially compared to those advising her to be more assertive and challenge the friend's behaviour. Sometimes people forget how little confidence they had when they were younger, or have very heavily rose tinted glasses on when they think about themselves.
I will freely admit that my previous comments were wrong as I simply hadn't quite appreciated the full circumstances, and there's a chance I might be wrong again. If I am, OP, please do correct me. The options I provided were totally unviable in the OP's situation, so I'd like to start again.
I think this boils down to one person who's happy to take advantage of, if not outright abuse, the good nature of those around them for their own benefit, and another who's being taken advantage of, who doesn't know how to stop being exploited without all hell breaking loose, and who's more terrified of all hell breaking loose than being exploited. This is someone who needs understanding, serious advice, and a reasonable strategy which doesn't expect her to turn into a tigress overnight - because that's just not realistic and it's not going to happen.
OP, hiding the car and claiming to walk is more of a temporising measure until you can deal with the problem permanently. Assuming it works, it will hide the problem, but for a relatively short period. You need to find a way of reducing the dependency of Susan on your lifts, and walking once or twice might certainly start that process.
What you need to do is arrive and/or depart by car but separately. Finding a reason isn't difficult but make sure you've got it in your head. In essence, you need to be arriving from or departing towards somewhere other than your home. You could've been visiting another friend, a relative, a shop/supermarket/store/similar, you could've been taking something to or collecting something from someone - either for yourself or for your parents (use your parents for excuses, parents are great for any type of excuse) - for example if you've bought something off eBay from someone working odd shifts and that's the only time you can collect, the possibilities are endless. You can also get creative, e.g., you're taking a large item(s) to somewhere for your parents tomorrow and the car is full, then fill the car with empty boxes. You can actually make it quite entertaining for yourself. You don't have to do this for every journey or both ways either. Start with a single leg; the arrival will be the easier one so you can start with that. It will be scary so don't underestimate how you'll feel, but do give yourself once you've pulled it off. Then the second, third and subsequent times will be easier. Once you've established that you giving a lift isn't a given, you'll find it much easier to say "no, I'm busy" without feeling the need to find an excuse or give a reason. Unfortunately, the first few do need little white lies, but those are no greater than saying that you're walking and hiding your car. In fact, you're far less likely to be found out than if you've hidden your car so they're safer and they establish the fact that you plus your car don't automatically mean a lift.
This is what I can come up with at 2 am after having had a general anaesthetic today so it may sound somewhat silly, but I think that this could be a possible strategy which doesn't involve direct confrontation yet stops you being taken for granted. I genuinely hope you read this, find it useful, and if nothing else, see that others can understand.