Jess recently donated her truck to a charity (I believe benefiting people with developmental disabilities). We haven't needed to have two cars at the same time once since moving to Seattle, so the extra insurance and registration costs were a total waste. Actually, we don't really need a car on a daily basis, though we have some semi-regular trips that aren't convenient for us on transit (Lynnwood Skate-and-Bowl, and to some degree our church, but only because we're lazy on weekend mornings). So I'm trying to figure out whether it would be smart for me to get rid of my car and go ZipCar.
ZipCar has a page comparing car ownership costs to its own costs. Clearly it's picked a very expensive case of car ownership costs that doesn't match my situation. I have a 10 year-old car that I bought with cash, so payments and depreciation are not close to the $288/mo. listed, and I have no finance charges. My car probably depreciates at about $50/mo. at this point, which will decay exponentially over its life (unless something weird happens). I spend about $40/mo. on gas, not $100. Also their estimate of parking charges doesn't apply to me — I don't pay for residential parking, and probably won't in the near future (parking for the first car in a household is essentially free in Wallingford). Their other estimates are probably about right (the insurance estimate is about what I pay, I didn't investigate the others very closely), so I figure I spend $267/mo. on my vehicle. Most of the costs are fixed time-based costs of car ownership, which are a pretty big waste for someone that drives as little as I do. That's why ZipCar can probably save me some money.
ZipCar has some advantages over ownership of my existing car (it can be bigger or truckier if necessary, or smaller if that's more convenient) and some disadvantages (unfamiliarity with vehicles, need to plan reservations and stick to the plan). Clearly it wouldn't be economical for vacations or driving to relay races (though it might work for relay race vehicles... would have to do some math).
As it is, I won't get rid of my car and switch to a ZipCar today. Not because it doesn't make sense right now, but because my employer might move offices within the next couple years, and we don't know where. It's possible that would force me to buy a car, which would blow all those cost-savings out of the water. If a meteor fell out of the sky and destroyed my car completely, however... I'd probably do it. If I had the ability to buy and sell cars at exactly their market value, without significant additional costs, risks, or effort related to the transaction attached, the distinction wouldn't be as important, but because I don't, the status quo has an inherent advantage.