My summer car is a highly modified SVT, and like the title suggests, it runs a full blown, watercooled desktop PC in the trunk...
Among its many, many uses, it has a USB OBDII module running fulltime, and DashCommand software displayed up front, which gives all the same readouts as the popular scangauge hardware.
I've never (until now!) ever thought about using it as a driving aid for efficiency...but I just might, when it pull it back out this spring.
Although, I'll admit, I didn't build it for efficiency!