[Ryland]

How often have you crashed your motorcycle over how many years with how many miles?
If you have never ridden a motorcycle before and are buying one with the idea of saving gas, then it's most likely a bad idea and you will spend more then you will save.
But if you have a motorcycle already and the gear to go with it and have a good record of riding without crashing or getting run over then it will cost you nothing more then putting fuel in the tank.
At that point it's a matter of looking at your own personal route, for weekend riding people tend to take routes that have less traffic because it is less stress, safer and it is an option, for commuting to work every day on a motorcycle you then have to look at your route, are you choosing a route that is safer but longer and there for going to use the same amount of gasoline?
The increased safety that you can gain from riding a motorcycle is that you can move out of the way of other vehicles easier, the big decrease in safety is that it's harder for other drivers to judge how far away you are and to see you at all and that when/if you get hit you have less around you.
But then if you look at motorcycle crashes you'll notice a pattern, they tend to happen to new or young riders, happen at night or in bad weather, they don't tend to be wearing a helmet and they tend to either be tired or have been drinking, or going to fast, all of those things are things that you have control of, other then the weather.

It's also depends a great deal on the motorcycle you have as to if it will save any gas at all as there are motorcycles out there that get 45mpg but like was pointed out, some scooters get great mileage and cost less, but they also go slower so once again you have to look at your route, is it all 25mph speed limits? scooters are also nice because most places you can park them for free on the side walk and at bike racks so if you are otherwise paying $10 per day in parking then a scooter really could be the way to go, but if you sit in traffic where people are often rear ended and have an otherwise ugly commute a scooter will not work at all and even a motorcycle at that point is going to be a poor choice.