How can this be possible?? Was I ripped off?

[jmbeam]

I have a 1995 E300D and the manual says it holds 18 gallons. I filled up for the first time and I put in 20.6 gallons of fuel. Was I just hosed by a Sunoco gas station that is not on the up and up? It was the first time I used this station because it was usually so much higher than any other station. However the past few weeks it has been selling diesel for $4.09. roughly .16 to .25 lower than the competition.