The above is the conversion for kilometers to kilometers per hour. Kilometer, as well as miles per hour are ratios of distance over unit of time. To elaborate on this, to say that you are going 81.6 km/hr means that, assuming you maintain a constant speed, you have traveled 81.6 kilometers in an hour. Kilometers is a unit measure of distance and hours is a unit measure of time, thus to get a speed (what is known as a velocity by us science-y types) we divide the distance over the amount of time it took to go that distance. The general equation used to find velocity looks like this:
V is equal to velocity or your speed.
D is equal to your distance within a given time (816 km).
T is the amount of time it took to go a specific distance (10 hr).
If we plug in 816 kilometers for our distance and 10 hours for our time we get an equation that looks like this:
Since we're only dividing by 10 we can move the decimal to the left one place leaving us with:
To convert this to miles we can multiply 81.6 (km)/ (hr) by 0.621 since there are 0.621 miles per kilometer, so this leaves us with:
So we would be going about 50 miles per hour if it took us 10 hours to go 816 kilometers.
Hopefully that helps!