Why Tailgating (ah-hem, drafting) Is Good For Fuel Economy

Nerd Alert: I'm going to engineer out on you in a second, so don't freak out if I drop some buzzwords like "Thermodynamics" or "Surface Integral" or, well... okay--I won't actually use any of those words. False alarm. But seriously, I want to talk about drafting (or in some circles, tailgating) and I think it's about time that we get technical about it. So let's get started.

I'd venture to guess that most of us can remember back to good old driver's training where we learned to keep two car lengths between us and the vehicle ahead of us. And for most people (read: those of us that do not treat an automobile as an extension of ourselves), this is probably a good idea. I mean, safety first. So, from this point on, let's just assume that the drafter in question actually knows how to handle his/her car.

Alright, now that we have the technicalities out of the way: Why do we draft in the first place? To me, the reasons are two fold. First, we tailgate to make our presence known. Until we can broadcast our thoughts into another persons car--perhaps something like "Dear Sir, Please move into one of the right most lanes if you plan to travel at a speed that is 20 miles per hour less than the posted speed."--tailgating is our next most readily available option to make our opinion known on the road.

But what about when that person in the late '90's Prius and no muffler doesn't get the message? Is there any benefit remaining? Most would say, "Just pass on the right, Tommy." But wait one second, you guys. I contend that there is some benefit to sticking behind this individual and practicing the respected art of drafting. For a brief introduction, take a look at Mike Rahal's video on how extreme tailgating has gotten in the context of NASCAR.

So let's take a closer look at the details (look away if you're squeamish when it comes to math). As already alluded to in the video, the whole basic concept of drafting is reducing the amount of drag on your car by taking advantage of the airflow of the car in front of you.

The symbols in this equation (working from left to right) are: the force of drag (FD), the density of air (rho), the velocity of the car relative to the air (v), the coefficient of drag (CD) and the cross sectional area of the car (A). So when you draft, you're using the airflow coming off the car in front of you to reduce the velocity of the air acting on your car. Therefore, the force of drag on your car is significantly reduced! The reduction of drag is especially large as the air speed is reduced since the force of drag is related to the velocity of the air acting on your car by a square.

Let's do just a small thought experiment. Let's say (just to make the numbers easy) if you were to be travelling alone on the highway going 100 m/s (v) and have a cross sectional area (A) of 10 square meters (all other factors are 1), the force of drag is equal to 50,000 N. Now, if you start drafting behind someone such that v becomes something like 50 m/s, the force of drag drops to 12,500 N. So, if the velocity of the air around you drops by half, your drag force is a quarter of the amount it was before!

I'll spare you the free body diagram, but if there is only 25% of the force on your car, you'll only be using 25% of the fuel you'd need if you weren't drafting!

So, go on--draft in the name of fuel economy (and physics).