I followed a large delivery truck for about 10 minutes today and my MPG shot up to 75! I kept the normal safe following distance behind. I thought "drafting" only occurs when you're right on someone's bumper.

Has anyone else seen this phenom? Any aerodynamic (or other) engineers care to comment?

 

Right after the Honda Insight was introduced to America, there was a bet to see if anyone could get better fuel economy than Honda. The editor of Car & Driver drove his Insight inches behind a Ford Explorer and got around 125mpg.

If would be interesting to find out if drafting could allow Prius drivers to sustain EV mode above 40mph....

 

You don't say how fast you were going, was this on the interstate? There are a lot of places around a large truck where you can be affected, positively or negatively.

• One is just forward of the truck cab, in the next lane. The air the cab is pushing out of its way, pushes againt the back of your car - a tail wind.
• If you are beside the truck and can see the driver in the side mirror, you are behind the trucks 'wave' and have a wind break (and potentially shade from the sun ).
• Right on the truck's tail WHERE NO ONE SHOULD EVER BE!!!! Yes, you get sucked along by the truck. Yes you get great mileage and Yes, anything bad happening ahead is unknown to you. May you rest in peace and hopefully not injure anyone else with your foolishness.
• Further back there can still be a wind break but now you are close to 'eddy land'. Just as the truck forced the wind away, it is sucked back in behind the truck. You can feel when you are in it either behind or beside the truck. You don't want to be there, not at all advantageous.

In good weather I use a minimum 2 second rule with cars, 3 with trucks. If I can't see what is in front of them, I'd like the extra reaction time, thank you.

 

Talk of drafting for mileage worries me.

I'm not an engineer but I do have very relevant experience. I used to race bicycles and during training rides sometimes drafted semis at up to about 60-ish mph (had a speedo some of the time). On a bicycle you needed to stay within 2-3 feet of the rear tires (depending a bit on wind strength & direction and type of trailer, see below) or you were into the turbulent zone and gone. I'd try to ride behind the outer rear tire so I could peer around the corner and whip outside the box if he hit the brakes. This worked especially well in a cross-headwind, which would shift the "pocket" slightly to one side. Laden semis were preferable because they changed sped gradually, but of course with a closed box you couldn't tell before you hopped on.

I would NEVER try to draft behind a random truck on the highway in my car. As bruceha_2000 points out and according to my own experience, unless you're REALLY close you're in the turbulence, so you don't get the full advantage and may even lose. And moving a car sideways to clear a rapidly slowing truck from that close would take far too long.

I don't draft on my motorcycle either, but on it I definitely get slapped around by the turbulent air so I try to get by semis on the Interstate ASAP.

I suspect a lot of people THINK they are drafting a truck but actually are sitting back in the turbulence - I could feel the pocket on a bicycle and it's a lot smaller than most people think. So they may be putting themselves at risk for little or no advantage, unless they are REALLY close.

For the record:
(1) moving vans and motor homes worked best because they went all the way down to the pavement
(2) bus exhausts are in the rear (gag!) and school buses had the added issues of stuff flying out the windows and imps making faces at you from the rear window
(3) flatbeds, tankers, bulk haulers, and car carriers were fairly useless
(4) 6- and 10- wheelers were too skittish
(5) I once made the mistake of jumping on behind a cattle truck (well, you can't see the trailer type when it's coming up behind you)

 

It was on a state route, going 55-65 mph. I stayed about 4 car lengths behind the truck, would that put me in the turbulence?

You mean the truck's side mirror, right? Then I would be just behind the front of the truck.

Ah, yes, the young and foolish days! No more.

 

The only situation at normal highway speeds (up to 70) where I've drafted off trucks is in the case of a strong, sustained cross-wind (winds offset 30 degrees from straight ahead are the worst as they generate air resistance with both the side AND the front surface areas of the vehicle). You can then drive in the lane adjacent to the truck and be shielded from the wind.

In one case however where I was driving in my friend's Nissan Sentra on the I-40 in Arizona, we managed to draft off a truck driving 90mph down a gradual grade (far faster than terminal coast velocity) for a couple minutes. At those speeds, air resistance is double what it is at 65, so the size of the "wake" behind the truck is much larger, allowing aerodynamic drafting from far enough away to still preserve visibility. The effect is also much larger, dropping the RPMs by several hundred. Other cars passing alongside (at speeds of ~100) would actually create a strong "push" on the side of the car, an effect not very noticeable at 65mph.

That trip was somewhat crazy, for a few hours, the only driver we saw under 80mph was in an RV, and was overtaken by multiple trucks... I was also surprised that we averaged 32mpg at speeds of about 90mph most of the time, that's better than what my friend gets on average in mixed city/highway driving (27.5).