It's pretty tough to stick with motivation A) as my fuel price has dropped by 31.5 % , fulfilling C) - that is if I even had to pay for it ...
I'm currently using the 500 euro credit I got from a fuel supplier when I bought the GasDwarf, a VW up! on CNG.
At the same my fuel consumption has dropped (albeit in liters to kilos) by 22.5 % .
Tailpipe emissions with CNG are one or two orders of magnitude lower than the diesel I was using before, fulfilling B) .
I'm waiting on somebody to start producing biogas in Belgium, so I can change over to biologically / locally produced natural gas - motivator D) - which I'm trying to help happening.
While you can get biogas in the Netherlands or Germany, Belgium doesn't even have legislation or regulation in place on how to practically make it happen
If things stay the way they are for CNG in Belgium - even relatively speaking compared to other fuels - my VW eco up! is expected to pay for itself over the next 10 years, compared to keeping Hägar the Volvo running.
And that's on the fuel bill alone ...