Prefab subtypes - Any methods considered good practice?

I would use the Factory design pattern here. Basically, you create a static Factory script, which you can then ask to create any type of car you have. This is especially useful if you need to instantiate lots of cars during runtime, but less so if you want to place them in your scene via the scene view, you'll need the prefabs for that.

 using UnityEngine;
 using System.Collections;
 
 public static class CarFactory {
 
     public static GameObject CreateSportsCar(){
         GameObject result = new GameObject("Sportscar");
         //add whatever meshes or other components here
         return result;
             //Or if you do create prefabs, just instantiate them here instead
     }
     
     public static GameObject CreateSUV(){
         GameObject result = new GameObject("SUV");
         //add whatever meshes or other components here
         return result;
     }
 }

As for the functionality for your cars, I imagine you mean code. I would create an abstract class Car, which is then inherited to all other types. This way, you don't have to go through all scripts if you just want to change a default value in all cars.

 using UnityEngine;
 using System.Collections;
 
 public abstract class Car : MonoBehaviour {
 
     protected float speed = 5f;
     protected int numberOfTires = 4;
     
     public abstract float GetWeight();
     
     public int GetNumberOfTires(){ return numberOfTires; }
 }
 
 public class SportsCar : Car{
     
     float weight = 200.5f;
     
     public override float GetWeight(){ return weight; }
 }

Now, if you want all your cars to have only 3 tires, instead of changing it in 15 scripts, you change it in Car. So you could create the prefabs too.