Generally, the closer you are, the more benefit you'll get from drafting. Being several car lengths back puts you right in the "buffeting" zone of the air coming off of the truck (I think, I'm no aerodynamics expert). The one time I drafted a semi on a level interstate, I was within a car length and my instant mileage pegged out at 120 MPG.
Keep in mind -- this is not a safe practice.
|