so basically, i have a car and the problem is that players can push the car around when it should be unmoveable, similar to Anchored.
What ive done is ive made the car anchored when no one is in it, however when someone is in the car, another player can push it around.
How should i make the car (or part) un moveable like the Anchored Property, but not actually anchored ? 
You should try to make the car “heavy” using physical properties, the key property you want to change is the density. Considering you made a car, I assume to wheels would be movable depending on your car if it uses Roblox physics or different kinds of method.
About the physical properties: PhysicalProperties
For changing the properties: BasePart.CustomPhysicalProperties
I’d make an if statement saying if player is in car then anchored == false else anchored == true
Imagine if the player jumps out of the car while falling, then there would just be a car anchored midair.
Lol I didn’t think about that 
already tried that
ive solved this by not letting players leave the car while in motion
ive actually also tried this, and it works however it makes the car almost undriveable and makes it kinda sink into the ground
THE CARS ENTIRE MASS IS BASED OFF 1 PART NAMED "MASS"
There is a “sweet spot” on finding the right density that it doesnt fly around like a feather or sink and weigh down like a rock…keep adjusting until u find the right density.
Boats are the same way.
Did you try changing MORE PROPERTIES especially on the springs of the car and it’s torque and stuff?
edit: its the SpringConstraint property.
For the engine I think your script or driver seat has some properties regarding the engine torque and maxspeed.
I tried adding springs constraints and multiple other attempts it seems that nothing changed; the car still sunk into the ground
ill send you the file to the car so you can try to find whats wrong with it via DMs