It's important to keep in mind that Red Muffler, a regular Class-C Hero was rated at 100 points by Child Emperor's physical strength measuring device. In comparison, Fubuki's non-enhanced strength was 19, the average HA staff member (working adults) was 22 and Sitch (an adult male who has no known physical issues) was too weak to be properly measured. And while this device wasn't the most reliable, that was due to being unable to measure/couldn't measure those who were:
A: Too weak
B: Too strong
C: Have a non-physical source of strength (Psychics and probably stuff like Fighting Spirit)
D: Cyborgs/Robots
So, when it was properly used, the results should be acceptable to use.
While this does not guarantee Mumen Rider is super strong, apparently (and I did not know this) he fought and injured a monster in the Sonic audiobook which could tank police firearms and damage concrete with his fists at max power. Taking that into account, and the fact that a comparable Class-C hero is roughly 4.5x stronger then the average HA staff member who are already much stronger then your average citizen (apparently), you could form a scaling chain with kid Saitama, especially since he was at a similar level of strength to his bullies, who in turn were weaker then the police.