Drafting a big rig - Fuel Economy, Hypermiling, EcoModding News and Forum

ChristyRZ · 01-02-2010, 04:58 PM

I mentioned it in my intro post, but my clearest success story was on a trip that normally requires I stop for gas halfway through.

I made the entire trip on one tank, mostly due to sitting behind a big rig, 5 miles below the posted limit, for about 2 hours. Fortunately he was going the same way I was and never exited, and kept a steady pace.

It doubled my mileage on that trip, at least in theory. I don't have the exact numbers, but I remember being so surprised when we were back to our home city and hadn't needed to stop (it's a long trip we make frequently, and we know how often we normally stop).

I figure...those rigs are guzzling enough gas to power them along the highway...I might as well share some of that power in the form of a draft!

vtec-e · 01-02-2010, 07:09 PM

Thats a wind up right? WE DON'T DRAFT!

RobertSmalls · 01-02-2010, 07:41 PM

Quote:

Originally Posted by vtec-e

Thats a wind up right? WE DON'T DRAFT!

Huh?

In the absence of a crosswind, you get a FE benefit even three or more trailer lengths behind a tractor trailer. As long as you keep a safe distance between you and the truck, drafting is a great idea.

user removed · 01-02-2010, 09:40 PM

Here are your choices.

I95 north between Richmond and Washington DC. Speed limit is 65 and three lanes all the way with more when you get close to the metropolitan areas.

Left lane-if you don't average 80 MPH you have some SUV 6 feet from your rear bumper and they will pass you and cut you off with less separation than you could measure with a yardstick.

Center lane-if you don't average 75 MPH you have cars and big SUVs passing you on both sides with the potential of a simultaneous merge right in front of you and a nasty pileup.

Right lane- lots of big rigs going between 65-75 MPH where you can find a good drive doing the speed limit or slightly over and stay about 100-150 feet behind him, with the shoulder (paved and wide enough to drive on easily) as your escape route if something happens with the idiots driving stupidly in the other lanes, or a large piece of tire debris appears under the truck in front of you.

If you allow more separation between yourself and the truck in front of you, you invite the idiots to pass the car in the center lane by cutting you off by a couple of feet separation and passing the car in the middle lane on the right (illegal) using the separation distance you leave between the truck in front of you as a passing lane.

Your last option is to take another route that takes an additional 20 minutes in the 2 hour drive, and you run the risk of falling asleep at the wheel, or getting hit by a deer if its dark outside and killed.

I choose the right lane on the Interstate when its dark, or early in the morning when the traffic is less insane. I use the slower route when its light enough for me to see the deer before I get nailed.

In either the right lane at 65 MPH average of the alternate route at 55 MPH the mileage is basically the same. The alternate route is like driving on a different planet, with so little traffic that you have a hard time staying awake if you have had a long day.

The draft option (right lane) is good for an additional 10 MPG in the CVT Insight, with separation of 125-150 feet (more than anyone around you in any lane).

regards
Mech

roflwaffle · 01-02-2010, 11:11 PM

The problem w/ a ~125-150ft interval IMO is that it can take ~130ft before someone even starts to slow down.

ChristyRZ · 01-02-2010, 11:57 PM

So...maybe 'draft' is the wrong word.

But I'm pretty sure that someone can be behind a slow-moving tractor trailer (ie: 60 in a 65) in the right lane, follow at a safe distance, and still be able to benefit from the wind break.

And because this particular truck was going below the speed limit, even with space in front of me, no sane person would jump between me and him, since he was so pokey.

I read the 101 tips after I posted this, and noticed the 'no drafting' policy, so if I upset the apple cart, I apologize.

k.civic.f4i · 01-03-2010, 04:59 AM

the potential for an accident is always there regardless of if you are drafting. i figure... if something is in the big rig's path, it will probably plow through it(unless it's another big rig).

people also argue that the trucks can stop abruptly but its highly unlikely that they could slow down faster than whatever you are driving(passenger vehicles)

i once made a 92mile round trip drafting and achieved 46mpg (calculated), whereas my average mpg is around the mid to high 30s.

in CA the big rig drivers go through A LOT of PAID training to get their license which includes behind the wheel.

the risk is on you.

The Atomic Ass · 01-03-2010, 06:10 AM

Quote:

Originally Posted by k.civic.f4i

people also argue that the trucks can stop abruptly but its highly unlikely that they could slow down faster than whatever you are driving(passenger vehicles)

People are also wrong sometimes.

The only semi rig that can stop faster than a car with functioning brakes is the semi rig that plows into a reinforced brick wall.

Now another rig BEHIND you might be a safety hazard.

pstrbrc · 01-03-2010, 09:40 AM

Actually, there are risks that go beyond braking distances.
#1. Trucks have a huge draft, and they suck up/throw up a lot of trash.
#2. Trucks can drive over road hazards that'll take your undercarriage out. So if it's heavy enough for the truck's draft to leave on the ground, it's gonna break something going under your car.
#3. One word: "Gators". Semi tire treads.

When riding motorcycles, (no windshield or fairing) drafting behind a semi just made the trip easier. Until I saw a gator take out a Crown Vic's front fender. Maybe with a very "aero" car one could expect minimal damage, but what about the undercarriage?

Now, if traffic is bumper-to-bumper, and you have to be behind something, and that just happens to be a semi...