First of all, you haven't said anything about what direction the car is going, so we're only talking about speed here, not velocity.
Take these two fractions: (1 hour/3600 sec) and (1000 meter/0.621 mile)
Each of those fractions is exactly equal to ' 1 ', because it has the exact same thing on top and bottom. So I can multiply any quantity by them, without changing the value of the quantity, only the units.
I'm going to take your 80 mile/hour, multiply it by those 2 fractions, then simplify everything:
Speed = (80 mile/hour) x (1 hour/3600 sec) x (1,000 meter/0.621 mile)
Speed = (80·1·1000 / (1·3600·0.621) (mile-hour-meter/hour-sec-mile)
Speed = (80,000 / 2235.6) (meter / sec)
Speed = 35.78 meters per second
