Honestly, everyone replying to this thread must be at work to have so much time for their long answers!

Well, I am, anyway.

The use of different measurements is easy:

Human height: Imperial
Weight: Metric
Newborn baby weight: Always Imperial
Newborn baby size: Always Imperial
Speed of vehicle: Metric
Speed of vehicle at speedway: Imperial
Measuring things precisely: Metric (mm)
Measuring things roughly: Imperial (there're no inches in Metric)
Measuring precision engine parts: Imperial (so you can say things like " 3 thou")
Describing the weight of really big things: Tons or tonnes, it doesn't matter as everyone just says "tunns"
Fuel capacity: "F" or "E"
Mileage: It doesn't matter as it's always better than the guy you're talking to

Actually, I used to use Imperial for fuel calculation and then changed to l/100km when I moved up here, because everything is so far away it's always hundreds of k's anyway.