[robotosan]

I generally just multiply miles * 1.5 for a rough approximation.

[FartyMcFarter]

Adding 10% of the original number to that is often easy as well, and makes the estimate very close to the correct conversion.

[hestipod]

Works great for kg/lb conversions too.

kg x2 + 10% = lbs

lbs /2 - 10% = kg

I've yet to find a fast one for C to F temp conversions though. It takes a bit longer to do the 9/5-5/9 + or - equation in your head.