As I've mentioned previously I do some drafting of trucks to increase my gas mileage. I did some estimations and calculations, described in that post, that indicate that, done with extreme caution, it could be safe and that it should be effective. But how practical is it? It's turned out that I'm only able to find a truck to draft between 10% and 15% of the time I'm at freeway speed. Trucks are frequently going too fast for my fuel saving regime, thus leading to the question of what the break even point would be for the fuel used in speeding up to draft a fast-moving truck versus maintaining a leisurely 55 m.p.h. with no truck in front of me. More on this later.
But the fact is, there is rarely a suitable truck around when I need one. I look for trucks with the following characteristics, just based on intuition: low trailer; as square a back as possible (preferably not a milk or cement tanker); not hauling rock, dirt, etc. (no dump trucks); maintains a reasonably steady speed; doesn't do a lot of lane switching, I guess that pretty much covers the candidates. But at 55 m.p.h. I'm not doing much passing of them, so I have to wait for a truck that has the listed attributes to pass me. It is rarer than I would have guessed, though if I see a likely prospect in the rear view mirror, I can slow down to let him catch me.
So when does it pay to speed up to draft? There are two aspects to this - the fuel used in accelerating to a new speed, and the balance between the reduction in drag from being behind the truck (this would be in the density term in the drag equation) and the increase in the speed. Additionally, road loads would increase by a small amount, but I assume this to be linear, and thus the increase in road load force is compensated by the increase in distance covered.
There will be a number at which my fuel savings from reduced density (the low pressure zone behind the truck) will be overcome by the increase from the speed term, since it's squared in the drag equation. For the purposes of this post, I'll ignore the fuel used to get up to a higher speed - this fuel is used to increase the kinetic energy of the vehicle, I'll assume I can recapture this energy (though of course I can't, at least not with 100% efficiency).
I have to use the figures in my previous post on drafting to calculate the reduction in drag to attribute to the truck's wake, and calculate from there. Using my best estimate of the increase in gas mileage while drafting, from 21.5 m.p.g. to about 25 m.p.g., I can calculate that air density is decreased by about 14.0% (from about 1.16 kg/m^3 to about 0.998 kg/m^3). Again, unless I receive a huge outcry demanding the details of the mathematics involved, I'll only outline the process and give the results.
Plugging these density results into the drag equation and realizing that I want to minimize fuel/distance (gallons per mile) = energy/distance = work/distance = force * distance/distance = force, I merely need to determine when drag using the decreased density behind the truck but an increased speed exceeds drag at normal density and 55 m.p.h. As it turns out, that speed is a little over 59 m.p.h. So if I have to go faster than 59 m.p.h. to draft a truck, I will lose fuel efficiency compared to driving 55 m.p.h. in the clear.
Thus, the battle becomes one of finding a truck with all the characteristics listed above AND that is not going faster than 59 m.p.h. This has turned out to be extremely difficult. As I refine my data, I'll revisit these figures.