For example, speed of sound is 750 mph, very approximately. The distance 750 miles is between 500 and 1000, putting it at rank 18 distance. Time at 1 hour is at rank 9. So Speed/Flight 9 is roughly the speed of sound.
That said, this only accounts for single move action movement. Using both standard and move to move doubles your speed, which is just one rank higher, and Extra Effort can also increase your speed rank by 1 for one round. So if you want someone who's absolute top speed is the speed of sound, when they put absolutely everything into it, that's Rank 7.