Not sure about the situation in France, but in Germany the diesel/petrol difference is much older than any CO2 concerns. Here, Diesel is taxed less than petrol

Not sure about the situation in France, but in Germany the diesel/petrol difference is much older than any CO2 concerns. Here, Diesel is taxed less than petrol, originally as a measure to support agriculture (who traditionally use Diesel engines on the field). There's a slightly higher tax on the car itself with a Diesel engine. Together this makes Diesel cheaper if you're using your car _a lot_, and so vehicle fleets (corporate, car rental, ...) are all Diesel based. That's hard to roll back politically (since it's cutting into expensive investments of huge organisations).