but -40 celsius and -40 fahrenheit are the same thing!

ten degrees difference celsius = eighteen degrees difference fahrenheit. but i suppose that only really helps if you care about thinking in both systems and are good at simple pointless mathematical operations. such as i. i kind of grew up using both systems fairly interchangeably, despite being american.

i measure driving speed in mph, but driving distance is time-based anyway (that place is 20 minutes away, per se). this is more logical than measuring driving distance in miles or kilometers, except on a small scale when, say, you're on a highway and you see a sign saying "Boston 150" meaning Boston is going to be somewhere between 2 and 2 1/2 hours more driving time - IF you were able to go between 60 and 75 miles per hour for the entire remainder of the drive. which is not something you're going to be able to do, because for one your destination is likely not exactly 150 miles away, and for another thing your driving speed will fluctuate once you get closer to the city and have more traffic and have to make numerous turns and stop at traffic lights and other such things. and since the metric and imperial units of time are equal, there should be no argument about how to measure travel distance at all, since as long as you choose one system or the other, the units cancel out leaving you with something everyone is familiar with, namely time. (distance / rate = time)

straight distance, not related to travel, is typically used in a place where a simple conversion is possible. (1 mile = 5280 feet = 1609ish meters. we can all multiply, right? that's all this is!)

weight and mass. nobody uses Newtons because they're impractical in a real-life, non-scientific setting. (how many pounds to a Newton? don't know? exactly.) and when you're talking about weights on this planet, using kilograms as weight is applicable as long as you convert correctly (approximately 2.2 pounds per kilogram).