so...

power = acceleration * (mass * velocity)


weight and speed (mass velocity) would be known.. but if maintaining a set speed acceleration would be 0..... and the end result of that formula to be zero, I would imagine acceleration would have to do with friction of tires,air?

it is easier to use negative acceleration as kown as braking or coast to get those numbers in a time frame. It usually done from 60mph coasting to 50mph and taking the time as exactly as possible.
I will copy the complete process, if you need it. It will be tommorrow in your messages.