Alright, so I develop for a group and the owner keeps pushing me to make a speed radar for his police vehicles, my problem is I have no idea how to go about it...
I've had a few ideas like making the vehicles store a speed value in their car as it changes, but I can't change the coding of any of the civillian vehicles, so that won't work.
I have wondered if there is a way to pull a player's speed, but I have no clue about that.
Any and all suggestions or ideas about what to do for this will be a huge help to me... Anything will help related to this issue.

3 comments
Velocity is distance/time. Just calculate how much distance the car goes in x amount of time.
As Phlegethon said, use the formula for velocity. I generally just run a spawn function which holds an old point (p1) and a new point (p2), and check their magnitude. Then divide the magnitude by the spawn function's interval rate.
You could use :GetRealyPhysics() - https://developer.roblox.com/api-reference/function/Workspace/GetRealPhysicsFPS