Rigidbody.GetRelativePointVelocity

Скорость относительно жесткого тела в точке relativePoint.

GetRelativePointVelocity будет учитывать angularVelocity жесткого тела при расчете скорости.

using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour
{
    public Rigidbody rb;

    void Start()
    {
        rb = GetComponent<Rigidbody>();
    }

    // Get the velocity of a wheel, specified by its position in local space.
    // This method assumes that the wheel is a child of the Rigidbody, or that the wheel translates relative to the Rigidbody. 
    Vector3 CalcWheelVelocity(Vector3 localWheelPos)
    {
        return rb.GetRelativePointVelocity(localWheelPos);
    }
}