Different govts get their fuel tax in different ways. We are just about to change our car from diesel to petrol. Road tax is €280 for the diesel car and €67.50 for the new petrol job. To make up that difference we would have to drive circa 30,000kms a year in a diesel motor - we do about 10,000, hence the change. Fuel prices change daily, sometimes twice daily over here with, usually, a 20% premium on petrol, so we pay on average €1.00 for diesel and €1.20 for unleaded. Sundays and Mondays are the cheapest times to buy and there will often be a 10% difference between Monday and Tuesday. Queues, alway, on a Monday.