how to define a speed of a vechile seat?

Issues

• Speed is not a property of VehicleSeat objects. What you should be referencing is Velocity.

• Consider when the property changes, rather than just updating the gui once. You can do this with the Changed event.

Note :: GUI code should always be in a LocalScript. There is no reason for the server to handle the client's UI.

Code

--Use WaitForChild since client code may load faster than the server
local vehicle = workspace:WaitForChild("VehicleSeat")

--Use `Velocity`
function match()
    --Index the magnitude member to get the length of the vector,
    --rather than the axis values
    script.Parent.Text = vehicle.Velocity.magnitude
end

match() --Match initially
vehicle.Changed:Connect(match) --Match upon changing