Rates of implementation

#1
Hello all,
I've tried to find this elsewhere but can't seem to determine how to do it. I want to determine the implementing rate of GPS technology in vehicles. I know the number and percentage of new vehicles sold each year and the number and percentage of new vehicles sold each year with GPS technology. Is it a common rate=rise / run situation?
E.g implementation rate = (% new vehicles with GPS at time 2 - % new v with GPS at t1) / ( year at time 2 - year at time 1)
Any help or direction to texts would be appreciated!
Thanks,
Kwak