How close do you have to be driving behind a semi-trailer on a highway to achieve any significant improvement in fuel consumption from the decrease in air resistance?
I would venture “none.” There’s not a vacuum behind a semi; you still have to pass through air. It could actually be worse to follow a semi – the air could be turbulent in the truck’s wake.
On the other hand, if you’re combatting an 60mpg (100km/h) headwind, you may have a point.
But, more importantly, what’s a “significant” savings in fuel consumption? Will 3MPG difference really change your life? Think about it: let’s say you get 20MPG, but you could get 23. You drive 15,000 miles per year. You pay $2 for gas per gallon (we’re pretending). That’s $1500 a year at 20MPG, and $1304.34 at 23MPG, less than $200 measly dollars per year, only a few bucks a week.
Now, if you want to buy a Prius (what, 70MPG?) versus a Excursion (~12MPG), then you’ll see significant savings.
(Then, when you’re driving the Excursion you’ll be pissed at all the Prius drivers tailgating you, causing tail-drag, decreasing your fuel economy).
Ther must be a significant advantage in doing this, because I always see semis doing it to each other. On the overnight haul between Sydney and Melbourne, you’ll see four or five semis driving along nose to tail. Every few minutes, the end one will pull out, overtake, and slot in at the front. This way, the drivers share the advantage.
When driving at high speeds on a narrow road, if a semi goes past in the other direction, there is a noticeable delay (one or two seconds) before I feel the effects of the wind on my car.
I think it’s one of those trucker brotherhood things though. I don’t think they take too kindly to other vehicles doing it. They hate us car drivers enough already - don’t really blame them.
Truck/truck drafting has a much more significant effect than car/truck drafting, however. A car or other 4-wheel is more aerodynamic than the huge, boxy big rigs, and therefore drafting does not have too much of an effect… perhaps 1 or 2 mpg, and keep in mind that you’re quite literally risking your life by following so closely (you’d have to be quite close to pick it up, I think… ever watched a NASCAR race and see how closely those guys follow to draft? INCHES). The trucker can’t see you back there and if anything happens and he needs to brake… you’re under that trailer. Goodbye roof and head. I hope you’re happy with your 2mpg.
Anyway sorry to get sidetracked. What I was saying is that, first of all, big rigs are more visible to each other when they’re behind one another because of their length, thus making it safer for them both to draft. Second, there’s a more significant increase in fuel economy when a huge big rig catches the draft of another huge big rig simply because an 18-wheeler has to displace more air to begin with. That’s why they’ll occasionally form draft chains.
Nah. Nowadays they (in the U.S., at least) have a bar sticking down from the back of the trailer to prevent exactly that. (Who was it that did that? Jane Mansfield?)
My bad. Last time I saw something like that happen I was 9 (9 years ago). (Ok, so that was the only time I saw something liket hat happen… but I’d heard about such a thing happening twice before, with people my father knew driving the trucks)
Once when I was young and foolish I was driving from Austin trying to catch a flight out of Dallas. My '63 VW Beetle could only go about 55 mph, and after a while I realized I was going to miss my flight.
As one of those big semi’s passed me I snuck in behind his draft and found myself going nearly 80 as I recall. There was a significant amount of turbulence when I dropped back too far, but when I was right on his tail it was a very smooth ride.
Kids, don’t do this at home!
Another phenomenon I noticed during my VW days was how difficult it was to pass one of these monsters. Often I could get right up to the truck’s front door, but would then be held back by the turbulence it was creating.
Well, at highway speeds it doesn’t do a very good job. Seen cars wind up under the back trailer. Well, the bottom half of the car is under the trailer, the top half is behind the rest of the car, resting on the road. You can tell how many folks were in the car this way: one red splash seeping out from what’s left of the roof - just one person, the driver. A splash on either side - driver and passenger. A very heavy splash on one side - you had someone in the back seat. The roof in sitting in a red puddle - full load.
Small, wedge-shaped cars come off worst in this.
Besides the fact that it is not really effective, it’s dangerous. You can’t see anything past the ass end of the truck if you’re that close, so you’re practically driving blind. Plus, it irritates the hell out of truckers.
I used to drive a truck OTR. I hated it when people would do that. Kind of unnerving having some little car right on your ass, knowing that if you have to brake or change lanes fast the remora would likely end up in a wreck.
I used to do an evil little trick if someone was obviously intent on drafting me all the way across Texas or something. I’d yank down hard on the trailer brakes, making the wheels lock up. This won’t significantly slow down a fully loaded semi, so it’s not unsafe. But the remora sees the brake lights come on & a big blast of smoke from the rear wheels. Usually clears them right out. 