How can i make my vehicle decrease speed when not accerelating

Here is my code that moves the car:

using UnityEngine;  

public class CarWheelControls : MonoBehaviour {
    [SerializeField] WheelCollider frontRight;
    [SerializeField] WheelCollider frontLeft;
    [SerializeField] WheelCollider BackRight;
    [SerializeField] WheelCollider BackLeft;
    [SerializeField] Transform frontRightTransform;
    [SerializeField] Transform frontLeftTransform;
    [SerializeField] Transform backRightTransform;
    [SerializeField] Transform backLeftTransform;

    public float accerelation = 500f;
    public float breakingForce = 300f;
    public float maxTurnAngle = 15f;

    private float currentAccerelation = 0f;
    private float currentBreakForce = 0f;
    private float currentTurnAngle = 0f;

    private void FixedUpdate(){
        //Forward/Reverse acceleration
        currentAccerelation = accerelation * Input.GetAxis("Vertical");

        //pressing "space" gives currentBreakingForce a value
        if (Input.GetKey(KeyCode.Space))
            currentBreakForce = breakingForce;
        else
            currentBreakForce = 0f;

        // Apply accerelation on all wheels. Change to front wheel or rear 
        // wheel drive by deleting fronts or rears :)
        frontRight.motorTorque = currentAccerelation;
        frontLeft.motorTorque = currentAccerelation;
        BackRight.motorTorque = currentAccerelation;
        BackLeft.motorTorque = currentAccerelation;

        //Wheel stopping
        frontRight.brakeTorque = currentBreakForce;
        frontRight.brakeTorque = currentBreakForce;
        BackRight.brakeTorque = currentBreakForce;
        BackLeft.brakeTorque = currentBreakForce;

        //Steering
        currentTurnAngle = maxTurnAngle * Input.GetAxis("Horizontal");
        frontLeft.steerAngle = currentTurnAngle;
        frontRight.steerAngle = currentTurnAngle;

        //Wheel turning
        UpdateWheel(frontLeft, frontLeftTransform);
        UpdateWheel(frontRight, frontRightTransform);
        UpdateWheel(BackLeft, backLeftTransform);
        UpdateWheel(BackRight, backRightTransform);
    
    }

    void UpdateWheel(WheelCollider col, Transform trans){
        //Wheel collider state
        Vector3 position;
        Quaternion rotation;
        col.GetWorldPose(out position, out rotation);

        //Set wheel angle
        trans.position = position;
        trans.rotation = rotation;

    }

I would like it to decrease speed when I'm not accelerating, but I can't figure out how to do it.