WheelCollider.motorTorque
var motorTorque: float;
float motorTorque;
motorTorque as float
Description

Motor torque on the wheel axle. Positive or negative depending on direction.

To simulate brakes, do not use negative motor torque - use brakeTorque instead.
	// spin the wheel forward

collider.motorTorque = 10;
using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour {
    void Example() {
        collider.motorTorque = 10;
    }
}
import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

	def Example() as void:
		collider.motorTorque = 10