The very best way to save on gas is not to drive. One way to cut back on driving is not to own a car. That is a pretty drastic way to save on gas though. What about the times when you need a car, or having one would be a lot more convenient? This is where car sharing programs come in. These programs are a step beyond carpooling, where you share your personal car because the idea is you don't have to own a car at all.
Zipcar is a car sharing program that lets you reserve a car for a time of your choosing. Could be a couple of hours to get some groceries or a couple of days to go on a business trip. You can pay on different plans. Their basic plan has you paying $50 a year as membership fee and then $7 an hour for car time, or $69 daily. The car time price includes gas and insurance. Zipcar is responsible for keeping the fleet in tip top mechanical shape.
Clearly you can save a lot of money on the price of a car this way. If you don't drive much, you can spend as little as hundreds of dollars a year reserving a Zipcar only when you really have to. The tens of thousands of dollars you could have spent on a car stays in your pocket. But does it help you save on gas? Gas is included in the reservation price of the Zipcar, so moneywise it is difficult to say. You have to separate out the insurance, maintenance, etc. costs. It also depends on the fluctuating price of gasoline.
In terms of amount of gasoline used, you will save on gas if the Zipcar of your choice is more fuel efficient than a car you would personally own. That is impossible to answer. But big savings would come from you driving less. Having your own car that you can just jump into without having to reserve time encourages you to make a lot of gas wasting small trips. Using a Zipcar will motivate you to plan your trips. Combining many small trips into one or walking or using public transport for some things can reduce the miles you drive. And reducing miles driven is the very best way to save on gas.