Safe drafting and impact on mpg
 

I've been wondering how effective drafting behind a vehicle is and if noticeable results can be achieved at a safe distance. Obviously, if I'm 3" away from the bumper of the car in front of me, I'm going to be utilizing their pushing of the air out of my way better than if I'm 100' behind them. But we live in a world where the unexpected happens and you can't safely travel 3" from the back of the car in front of you.

So, is there a safe distance that anyone has documented measurable efficiency gains by drafting leading vehicles?

Safety is a huge factor. Any gains would be erased 1000x over if there is a single safety breach. Also It goes a little beyond that for me in the way of damage from debris from the leading vehicle. Windscreen, headlight, leading edge hood damage impacts from rocks and dirt will out weigh when it comes time to repair.

So, the consensus so far (with one post) is that there is no safe distance at which a drafting gain could be realized.

There is a benefit to using a big rig as a traffic blocker for you. Traveling at a safe distance behind one can allow you to travel at a slower speed without negatively affecting faster traffic as much as you would if you traveled slow without the rig as a blocker. People can see the rig from far away and maneuver themselves around it (and you) in a much safer manner than if they just came upon you suddenly in their way.

I have found very minor benefit from the drafting affect when you are at what I consider a safe distance. I use Scangauge to confirm this. So I don't drive behind a big rig unless I want to drive slower than the rest of the traffic.

i use the two- or three-second rule of following and noticed that drafting helps even at these distances. i only have a vacuum gauge so can't provide any delta-mpg...

Same here, in my Acura, around 1.5-2mpg diff in lean-burn, and 0.2-0.4 out of it.

In my (recently traded in) Dodge 1500, I saw a 4mpg difference. That's not a typo.

Safety is, of course, relative. You're safer at home on the couch. If, as I suspect, I can shed mph faster than the truck, right behind it seems a pretty safe place to be. But road debris is another matter; trucks shed big hunks of tire tread. You see them lying along side the road; they're bigger than a deer.

Yes, I have done it. I prefer a flatbed with a full-width half-height load—less buffeting.

My Jeep Wrangler will gain 2mpg/tank even with only 50% highway. But some of that is at a slightly slower speed. At a 3W distance(3 wrangler) I can hear a large reduction in wind noise and feel the resistance drop. Tankers are best. Any closer and may as well be towed. Rocks, blown tires, eh. What, you want to live forever? Keeps your reflexes sharp.

If you drive on California highways with any amount of traffic, you are already drafting "at dangerous following distance", and I'd like to say that it's not dangerous as there is no option but to follow at distances that reduce drag considerably :) Now I don't know if we have more accidents here than in other parts of the country (I suspect we do)...

I like the big Fed Ex trucks that go way low to the ground. Those are the only ones I find usefull for any drafting purposes. I feel it at about 50' at about 60mph. At 80mph i'll be about 100' and still feel the effect. I only do this if im on the interstate and in a hurry.

The drafting effect isn't as good as slowing down. So I mostly just drive slow and let everyone pass me.

Drafting at a safe distance can give quite a big benefit because it allows you to go slower. If I go slow (say 60 in a 70) then I can fine people not noticing the speed difference and coming up really fast before a "last minute avoid" lane change. If I put myself behind something obviously big and slow then this doesn't happen.

I also use the "2 second rule" to avoid piling into the back of anything, but also being that little bit further back means you can see "round" the vehicle in front more which means you can see obstacles quite a way ahead.

Back in the 80s I was following a dump truck at approximately a legal interval, when I observed a rock about the size of my fist that was stuck in its rear dual tires dislodge and bounce off my hood directly in front of me. Fortunately it bounced sideways, and knocked the outside rear view mirror right off the car. So in a sense, I am already living forever. :)

the best way I found to gauge distance is watch as the debris , leaves etc, get carried along in the trucks wake, if your there your good, can be a pretty fair distance, also someone mentioned buffeting another good indicator , roll down your window and when you feel the air popping in and out of the car ,I think your drafting . either way you've got to stay glued to his brake lights for reference, and if he should crash, you do too. most likely. probably works better on a pickup than anything, I've actually felt gains when someone tailgates me with the right shaped car, can't remember what kind but it happens.

Here is a video of a tuft testing I did a few months ago. Notice at 3:10 how the airflow is somewhat attached (center of rear glass) and becomes very turbulent and detached when I stop drafting the bus in front of me.

http://www.youtube.com/watch?v=TpFLbOzxWn0

I've found this to be true in my car, while in my truck, the trailers that have those "Kamm wings?" On them netted the best FE

Safe following distances change depending on speed, weather, time of day, traffic, etc. I wouldn't get closer to something just because I'm drafting it, in fact I'll generally give it at least as much distance as I would a car because a good truck is hard to see through. If it's big enough to be worth drafting, it's punching a big enough hole in the air that you can do it from a safe distance. Besides, P&G won't let you pick an exact distance anyway.

The worse your car's aero, the more drafting will help. I find that I can get an mpg boost if the rig is going at my speed, and I can hang on to one going around 10 mph more than I want to go without hurting that trip's numbers- but I've found that trying to catch up with one isn't worth it.

I draft a lot less than I used to, but I think that's because I'm pickier about drafting opportunities than I was.

I agree about the heavy traffic "drafting". Some of my best mpg has been from 25-45MPH speed-up then slow-down traffic on the freeway.