I have been wondering this myself. Diesel burns cleans, can be made from vegetables and the engines can also be used with methane. Not to mention the engines are fundamentally more efficient than gasoline. Not only just as a system, but our current use case of low idles and sitting in traffic, a diesel wipes the floor with gasoline. You can get a diesel in the us, but unless you get the vw, you're stuck with a 4 litre truck. I think another important question is why aren't we converting gasoline cars to run on methane? The conversion is less than 1000$ and can be done by anyone. Its a tank some hoses and a regulator and would bring prices down to about 2$ a gallon.