speed

i kind of want to gauge how fast im going when i do stuff. im really confused because i go on sites like the fish (obviously not the most reliable source but still) and people are like, yeah i hit 35 on my s9 going down some hill in my neighbourhood the other day, then i convert that too km/h and its like 55. so i know its not an important factor in longboarding when your just having fun or practicing but i still want to know. anyway, to gauge how fast people go could people please guesstimate or tell me how fast:

people were going at ashleys birthday bash on sunday (great event by the way)
kyle chin is going in the let go video
you need to be going to do a pendy without coming to a stop (i know this one is kind of shaky as it depends on weight but the other two are pretty good gauges i think)

if all this stuff is confusing and you have some other way of telling me how to gauge speed, beside getting a parent to drive behind me in a car (they would just decide they needed to restrict the speed i went), im open to that aswell.

thanks all.