I have never seen lower gas prices in the Keene area than at the Gulf station on Route 12, just south of a major Keene intersection of 12/10/101/Main St.
The station is small and is often too busy to get into (at least for my taste, especially when I'm driving the truck!). They "close" at 6 PM, but if you can pay by credit card and don't need a receipt, you can pump your own gas anytime! There is a machine near the door of the building that you swipe your card in, enter your pump # and fill 'er up!
I've used gas buddy to find cheap fuel. They depend on people, like Dawn, who observe good deals and, report back to them.
http://www.gasbuddy.com/