Let's talk kilometers and miles for a sec. If you're an American and you ever visit Canada, you might be thrown off by the speed limit signs. We know we were. That's because everything is in kilometers per hour instead of miles per hour when you get up north. Likewise, a Canadian might be super confused by America's mph signs—they'll look really slow compared to what Canadians are used to. So how do we convert between the two?
Let M be miles, and let K be kilometers:
K = 1.609M
M = 0.6214K
Since 
(this is one of those important formulas you should probably commit to memory), in order to convert a speed from miles per hour (mph) to kilometers per hour (kph), we multiply by 1.609:
To convert a speed from kph to mph, we multiply by 0.6214.
Sample Problem
A sign in Canada says "Speed Limit 110 kph (eh)." How fast is this speed in mph?
To convert a speed from kph to mph, we multiply the speed by 0.6214. Therefore, 110 kph is 0.6214(110) = 68.354, or approximately 68 mph. That should be just fast enough to power your way through a moose.