It used to be that the pump was just right next to the road. To allow easy access the fuel cap had to be on the side facing the side of the road. That's why cars in countries where one drives on the right-hand side of the road had their fuel cap on the right and for countries with left-hand side traffic the cap was on the left. For example, German cars have their caps on the right and Japanese on the left. Nowadays, of course, with cars being exported all over the world and with multi-lane gas stations off the road, this simple rule no longer applies.