My car has a part called "BackChassis" which holds both of the HingeConstraints (motors) from each of the back wheels. One named left, for the left wheel; vice versa. All objects in my 96 part car are unanchored, and I have qPerfectionWeld inside of the car.

Code:

local seat = script.Parent
local car = seat.Parent

local bChassis = car:WaitForChild("BackChassis")
local Left = bChassis.Left
local Right = bChassis.Right

seat.Changed:Connect(function(property)
	if property == "Throttle" then
		Left.AngularVelocity = seat.MaxSpeed * seat.Throttle
		Right.AngularVelocity = -seat.MaxSpeed * seat.Throttle
	end
end)

The seats MaxSpeed is 100. The motors have 500000 MotorMaxTorque to see if the car would budge. I'm not sure what else to do at this point or why the car won't move. If you need anymore information just ask or add me on Discord - Goat#2976