I've been doing similar experiments, however I have only been able to increase my mpg by 2.

Yesterday I decided enough was enough and went and bought a bus pass. I've used the bus before, but it was always a break even point and the inconvenience was too high if I wasn't saving money. Now that gas prices are so high I can save $70 a month by taking the bus. It would be much more savings except I have to drive 10 miles to get to the nearest bus stop. that's 20 miles a day of driving vs 50 miles roundtrip just driving in to work. Not a huge amount but the cost of gas is compelling me to cut where I can.
Perhaps I'll buy a bicycle for the good weather days and cut out the gas consumption even more. I need to check in to bike storage or what it takes to attach it to the front of the bus.

The other factor is the bus system got a lot better since I last rode it 4 years ago. They have increased routes, so I'm no longer stranded if I need to get to my vehicle in the middle of the day. They have also introduced cards you can load up with trips. If you lose the card you can cancel it, pay $2 and they give you a new card with all the trips you paid for from the old card. I no longer have to worry about losing my investment. Last of all the trips aren't tied to the month, so I don't lose them if I have a month where I need my car more and can't take the bus. These were all smart changes that were just recently implemented to make public transportation much better.