How do we define the true value of speed?

Imagine a car. Miles per hour is a measure of the distance I would've gone if I drove at a certain speed for an hour. It's not the actual speed I'm driving at that moment. If I drove 70 mph it means I would've traveled 70 miles in an hour, but I want to know how fast I'm going RIGHT Now, NOT where I'll be in an hour. All of these measures km/h and the rest seem to be the same type of problem. It's not even constant, no one drives at that high of a speed for that long, I want a measure of speed in speed, not in distance or time.