Jan-Feb 0.40%
Feb-Mar 1.50%
Mar-Apr 3.67%
Apr-May 2.70%
May-Jun 0.78%
Jun-Jul -0.38%
Jul-Aug 0.33%
Aug-Sept 0.91%
Sept-Oct -1.12%
Oct-Nov -0.96%
Nov-Dec -1.54%
Dec-Jan 0.08%
The data above refer to average monthly percentage changes in US gasoline prices from the year 1978 to 2008. One shortcoming is that these data are not weighted - I didn't consider gallons consumed. Still, notice that over the 30 year period, there is a decided tendency for prices to decline in the Fall. In 21 of the 30 years, there was a decrease in retail gasoline prices from September to October.
Source: Energy Information Administration, Department of Energy
Once again, the DUmmies are wrong.