Swapped from diesel to cheaper, cleaner burning CNG.
Got rid of the Diesel Particulate Filter misery - and got rid of the poisonous ultra-fine soot altogether.
118 g CO2/km with the Volvo, went down to 100 g CO2/km.
(that's without compensating for the biologically sourced CNG that I tank whenever I can)
Price per 100km came down from 6.13 to 0.62 euro.
That's temporarily though, as I'm mostly tanking using a 500 € CNG bounty I got when buying the car.
The actual price drop will be just shy of 50%.
The silly low price of CNG fuel does reduce my motivation to reduce fuel consumption even further.