I recently signed up for ZipCar, because, being a student at Stanford, I don’t need to use a car that often. When I need to get to downtown Palo Alto or Menlo Park or even Mountain View, I usually ride my bike. Same goes for grocery runs and taking care of errands; my bike usually suffices.

But every once in a while, a car is genuinely useful, and it’s kind of annoying not to have one handy. If I need to pick up a friend, or if a destination is outside a 5-mile radius and not readily accessible by public transportation (which isn’t the greatest here in the Bay Area), a car is a nice thing to have.

The problem with having a car is that there are so many things you have to pay for and worry about that owning a car gets really expensive really quickly. And this is discounting the fact that if you want to buy a relatively good, new car, you usually have to plunk down at least 20 grand.

If you’re an infrequent car driver like I am, owning a car makes no sense. The costs are too prohibitive and the return that you get for owning that car don’t offset the costs. But something like ZipCar is quite convenient, because you only need to pay for the car when you use it. Obviously it makes economical sense to go for a ZipCar over a normal car. But what if you’re a frequent driver? Does it make sense to buy your own car, or use a ZipCar? I wanted to do a cost-benefit analysis to find out.

There are three main things that you pay for when owning a car: gas, maintenance, and insurance. In order to see whether a ZipCar makes more economical sense, we have to take into account each of these three things.

A lot of the time, I use a car to go and pick my brother up at the San Francisco airport, from Stanford. According to Google maps, the distance of the drive is 23.6 miles one-way, or 47.2 miles round trip. Driving to the airport, picking up my brother, and coming back also takes a little more than an hour. Let’s use this trip as the basis for our analysis.

First up, gas. The ZipCar that’s parked closest to my apartment on campus is a Prius, so let’s assume that we get 45 mpg, and that gas costs $4/gal, which is frequently lower than what many gas stations charge in the Bay Area. The cost for gas for this trip is $4.20.

Next, insurance. According to insure.com (whose reputation I know absolutely nothing about and whose data I will not guarantee their accuracy), the average insurance costs for a Prius are about $1200 a year. This is in line with what one of my roommates says he pays for insurance for an older 1999 car. If we get a daily rate, that’s $3.30/day. We’ll assume that this is the only trip we’re going to make that day, so the full insurance cost is going into that trip.

Finally, we have maintenance. If you own your own car, you have to pay for all the repairs for your car. With ZipCar, it’s included in the cost of your trip. Maintenance can vary widely from year to year, but let’s assume, generously, that you only have $500 worth of maintenance per year. That translates to about $1.40/day.

Is there anything else we forgot? For me on campus, parking needs to be taken into account as well. Yeah, I could probably find some parking a few blocks from my place where I could park my car for free, but residents would be unhappy with me and it’d be annoying to not park right outside my building. A parking permit for me would cost $282/year, or $0.80/day.

This comes out to a total of $9.70. If the trip takes a little more than an hour, I’d have to reserve a ZipCar for 1.5 hours in order to avoid fees for being late in returning the ZipCar. At $8.66/hour (the stated rates are $8/hr, but when you go to make a reservation, it reflects the actual $8.66/hr cost), this trip would cost me $13. To be fair, the annual fee and the one-time application fee should probably be accounted for as well. That’s $50/year for the annual fee, and $25 for the application fee. That’s another $0.20 per day. (As a Stanford student, I actually have an annual fee of $35 and a $0 application fee.)

So the cost for this trip if you owned your own car would be $9.70, while the cost for this trip via ZipCar is $13.20. You’re paying an extra $3.50 for the ZipCar service for this single trip. $13.20 for that trip seems like a lot, but $3.50 is a lot easier to swallow.

Let’s do the same analysis for a different trip. Instead of going to the airport, let’s take a weekend trip to Yosemite. Let’s say you’ll leave Friday afternoon, and get back Sunday afternoon. I just priced this trip on the ZipCar website, and this is actually 2 days, not 3 as I would have suspected. The estimated cost is $129.90.

Yosemite is 189 miles from Stanford. This slightly exceeds the limit of 180 miles per day. The rate is $0.45 per additional mile, so you’d pay an extra $8.10 for this trip because of the excess mileage. Adding in the $0.40 for two days of ZipCar application fees makes this trip cost a total of $138.40.

If you owned your own car, at 45 mpg and $4/gal, gas for this trip will cost $33.60. Insurance will cost $6.60 for two days, and maintenance will cost an average of $2.70 for the two days. Parking for the two days (since we got an annual pass which we will have paid regardless of whether we parked on campus or not) will be $1.60. Total costs are $44.50. For the privilege of using ZipCar, you’re paying $93.90 for the service for this trip.

Clearly, ZipCar’s cost-effectiveness takes a plunge when you go on longer trips.