I think at 3-4 seconds you'd be too far out to make a difference. Did that on the way to BC the one year. Sped up to keep up with a passing semi, got better mileage with the trailer than if I hung back. But I was no more than a half semi length behind to get any benefit. Way closer than 2 seconds. Otherwise I'd rather avoid the stone chips.
There are 3 settings on my laser cruise. Close, Medium, Far. On the 401, far doesn't work well. You are waaaaayyy behind. I found out the hard way it's too far behind. With a truck or car way up ahead, the LS would start to brake...which pisses people off on the 401...braking for essentially no reason. Go figure!
The thing is...to keep a safe distance, those distances change depending on vehicle speed with the laser cruise.
I THINK at 80km/h, the distances are:
Far: 75 meters (3 second gap) LS430 is 5 meters long, so 15 car lengths.
Medium: 50 meters (2.25 gap) 10 car lengths
Close: 30 meters (1.35 gap) 6 car lengths
