To convert your Speed (ft/round) into MPH or KPH, apply one of these two formulae:
MPH = Speed / 8.8
KPH = MPH x 1.6
These formulae apply regardless of whether you’re running, flying, driving, or swinging on a web-line. If you can go 120 feet as a move action while riding your bicycle, then you can travel at 22kph (16mph), for example.
To get some real-world perspective, a Speed of 2880 ft. (90 ft. Move, x32 Run) is 524 kph/327 mph, which is just shy of the speed of sound, or Mach 1. So Mach 2, twice the speed of sound, is about 1047kph/655mph (90 ft. Move, x64 Run). Finally, 891,709kph/557,318mph (300 ft. Hustle, x16348 Run) is creeping up on 0.1 C, or 1/10th the speed of light. The idea that someone can move at these kinds of speeds by slapping their feet against the ground is, of course, ridiculous, but ridiculousness is sort of the point of a superhero game, so just enjoy.
If you have a non-standard base speed or you can sometimes move at different Base Speeds (for example if you can Size Shift), then please do all these calculations before you get to the gaming table. It’s only polite.