If cars speedometers read 5-10 over everyone would probably be doing the proper speed limit. amirite?

They often read over as things like tyre pressure effects it then a few over for good luck.

To my knowledge car speedometers read 3mph ahead of what your actually going

They do.

So i compared gps data from my cell phone and some gps device from cycling to the speedometer. And depending on the velocity driven the speedometer is missing 3-10 km/h.

So usualy only look on the speedometer my cellphone shows and than add like 5 km/h. So im still speeding.

In old cars they mostly do