It is a extremely unrealistic to expect you will be able to average 40 mph on this road, as I said the maximum speed is 45, but it is much less in many parts of the road. Your average speed is likely to be 30 mph or less. Mind you, that's assuming you never stop at any of the many viewpoints and other attractions along the route. Its a fantastic drive, but it isn't a road that works with speed, and 3 days would normally be considered about the minimum if you stick to the BRP for the drive you are talking about. Mind you, the BRP also isn't a road where you can easily get off and jump back onto a freeway to make up time.