The Greeks are pretty wild drivers. They love to tailgate. They use the shoulder as another lane. Be sure to pull over into the shoulder if they are passing you on a two-lane road. They also ignore the speed limits and think that stop signs are like yield signs. I was uneasy at first, but later found this style of driving rather liberating (if still rather dangerous). The good news is that because of the economic crisis, the roads will be relatively traffic-free. Also the "national road"--their version of a toll road--is now open all the way between Athens and Kalamata in the Peloponnese. My friends tell me Kalamata is now just two and a half hours from Athens Airport (it used to take all day).
The majority of gas stations in Greece are "full service." Pull up to the pump, and within a few minutes someone will come to fill your tank with gas. Hand them the keys if the gas tank is locked, and tell them how much gas you wish to purchase. I'd use broken Greek to say "30 euros" and the gas station attendant would answer in English--"Oh, you mean 30 euros?" Very rarely does the gas station attendant provide more service than filling your tank.
I'd read some of the websites and blogs about driving in Greece, but don't panic. It will be fine as long as you drive defensively.