In the UK, the balance is roughly 50/50, from a sample size of 100 (me going into streets and counting the cars). I did a simulation of petrol stations when I was at university, trying to work out optimal arrangements of pumps, so I needed to know if there was a bias towards the fuel cap being on a particular side of the car (there wasn't). This could be because in the UK our cars come from a wide-variety of places.