How to make the car move slower in the terraing and faster in the street

Make sure one of the objects that collide have the "Is trigger" option enabled in the inspector.

from there you could do:

 void OnTriggerEnter(collider other)
 {
 // your script
 }

Firstly

I recommend using Compare.Tag("terrain")

It uses less memory.

Secondly

If you want to do it with collision detection you'll need to tag your "street" and "terrain" then attach a similar script to your car (make sure all these objects have a collider component)

     void OnCollisionEnter(Collider col)
     {
         if(col.CompareTag("Terrain"))
         {
             maxSpeed = 100;
         }
         else
         {
             maxSpeed = 250;
         }
     }

That should work, however...

Hopefully this is a good suggestion :

Don't use colliders for this, instead use physic materials. Create a physic material for your street and for your grass, and play with those values untill you get the right effect. I recommend using gameObject.rigidbody.AddForce(); for your movement and letting the physic material do the rest.

Goodluck with your project :)

thanks both of you for trying to help me. I tried what u said but the problem is the fact it is not recognizing the car. I tried doing something smiler and easier like collecting the money. but it also does not work. the car just pass the through the money and its not collected. the money collier is on trigger. the car is tagged as (Player)

function OnTriggerEnter( other : Collider ) { if (other.gameObject.tag == "Player") { Destroy(other.gameObject); } }

I hope this makes sense?

thanks again for trying