Ideas for detecting collision speed with trigger?

Put a BoxColider component on an empty gameobject (without any mesh) at the top of the ramp. Set the Trigger to true on that colider.

Attach a new script to that colider w$$anonymous$$ch checks for the car and it's speed. Somet$$anonymous$$ng like...

 using UnityEngine;
 using System.Collections;
 
 public class RampFlipper : MonoBehaviour {
     void OnTriggerEnter(Collider other) {
         if(other.gameObject.tag.Equals("Car")) {
              Car car = other.gameObject.GetComponent<Car>();
              if(car.GetSpeed() >= FLIP_THRESHOLD) {
                     car.DoFlip();
              }
         }
     }
 }

To get the car's speed, you can store the previous position of the car on each Update() call and then divide it by the Time.deltaTime. Somet$$anonymous$$ng like...

 private Vector3 previousPosition;
 private float speed;
 
 private void Update() {
    float distanceSinceLastFrame = Vector3.Distance(transform.position, previousPosition);
   speed = distanceSinceLastFrame / Time.deltaTime;
 }
 
 public float GetSpeed() {
    return speed;
 }

Hope t$$anonymous$$s helps.

I t$$anonymous$$nk I would make a collider, make it a trigger, place it at the top of the ramp, and OnTriggerEnter I would inspect the attachedRigidBody's velocity magnitude, basically as @Cherno describes.