mpg was the measure used for the first half of my motoring life, and I see no need to change. Litres/100 km mean nothing to me. 300 miles mean something but to me 500 km is just a number. I use the units that suit my purpose, whether that be metric or Imperial. mm is a great way to measure, better than 3/16ths and the like, but prefer thou to microns when it comes to small measurements. As long as you use a measurement that suits the purpose. Not like the US builder that built the mansion designed by a French architect. The drawings were in cm and he built in inches!
My late father-in-law used a 3' wooden rule and told my wife, who was buying him curtains, that the window was 3' 18'' wide!