Specifically, celsius to fahrenheit and kilometers to miles.
I met a guy from Toronto who told me that, as a rule of thumb, 16 (C) = 61 (F). With that in mind, I noticed that for every one degree rise in celsius, fahrenheit would go up by two. E.g.
17 = 63
18 = 65
19 = 67
and so on. It’s not completely exact but it is very close.
For km to mi, an English friend told me to multiply by .6. (Or multiply by 6 and drop the last digit, if it’s easier to think of in that way.)
100 km (a number you’ll see on the open road) x 6 = 600 = 60 mi
50 km (a number you’re likely to see in town*) x 6 = 300 = 30 mi
To go from miles to kilometers, simply add zero to the end and divide by 6.
48 miles = 480/6 = 80 km
Again, it’s not completely exact but, within the 0-100 range, it is very very close.
*Canadians, at least in Vancouver, do not post speed limit signs in town anywhere near as much as Americans do (which is maybe why I thought Vancouverites were such bad drivers my first time there).