Dude you are looking at a year change in gas prices while Obama has only been in office six months.
This graph is only for San Francisco but I would surmise it can't be too far off the curve for the rest of the country. Gas is $4.61 in June 2008 and then $2.04 in January 2009 and is $2.90 today. So gases prices have actually risen under Obama.
Since you wanted to give him credit when you thought it dropped under his watch do you want to blame him now?
http://www.sanfrangasprices.com/retail_price_chart.aspx
Anyone will tell you the true measure of gas prices is year to year as it historically has followed certian trends during the year. Up in the summer, down in the winter.