Aside from the occasional crashed Mars lander, technical people in the U.S. get along quite fine thinking in the units best suited to the task at hand. I develop film in my apartment, and I use metric units while developing black and white but conventional units while developing color slide film — because the processes are documented in those respective units.
I don't know what kind of car Cranky Bill is driving, but if it's new enough, chances are he could crack the manual and figure out how to switch it to display values in liters/100km. My motorcycle does that, and my last car did it. I set my motorcycle to display temperatures Celsius, speeds in whatever the local country's unit happens to be, and fuel efficiency in mph.
Also, he might just look up the stats for his car on the Internet.
I don't know what kind of car Cranky Bill is driving, but if it's new enough, chances are he could crack the manual and figure out how to switch it to display values in liters/100km. My motorcycle does that, and my last car did it. I set my motorcycle to display temperatures Celsius, speeds in whatever the local country's unit happens to be, and fuel efficiency in mph.
Also, he might just look up the stats for his car on the Internet.
Bill sure is cranky.