Today's drafting experiment (Hypermiling/Ecodriving tip #38)
 

Today I dropped the kids off at the inlaws like I always do. It's a 61km (38 mile) round trip, 53% country roads with a few 4-way stops and lights, 47% highway.

I usually average 7.2 L/100 km (33 mpg), trying pretty hard - 7.0 L/100 km on the way there and 7.4L/100 km on the way home (elevation change). The Matrix isn't exactly an aerodynamic car, to say the least.

However, today when I go onto the freeway on the way home I decided to stay close behind a truck just ahead of me. It was going a bit slower than speed limit, 80-90 km/h (50-56 mph). I didn't even stay that close - maybe 1-1/2 to 2 seconds behind (a cop wouldn't have noticed), and suddenly what seemed like magic occured on the Scangauge.

The result of today's experiment ended up being 7.2 L/100 km on the way there and 5.2 L/100 km (45 mpg) on the way back, for an average of 6.2 L/100 km (38 mpg)! That's a 16% improvement.

I know this isn't exactly scientific with A-B-A testing or anything, but I definitely will be using tip #38 in the future.

If tip #38 is getting behind a truck, then it definitely works. The best tank I ever got was 38mpg in my Camry, and I did 55mph behind a truck the entire time.

Do the math, on the Hampton Roads Bridge tunnel, in either direction, with two lanes of traffic, 95,000 cars will pass over the same point in 24 hours during peak summer traffic.
There are 84,600 seconds in any 24 hour period. That's less than two seconds average separation for every day, for every one of 95,000 vehicles, when traffic is that high density.

I have posted the local VDOT traffic camera links where you can see how dense that traffic is at any given time. I find that drafting at 70 MPH (speed limit west of my home) gives me the same mileage as 55 MPH on US 60 which runs parallel to I 64 from Richmond to Newport News Va.

Even with that kind of traffic density, I have had other drivers pull over in front of me with less than a single car length of separation.

I employ a technique I call pulse-drafting when traffic is that heavy. On the downhill sides of overpasses, or other lesser downgrades, when I am behind a larger vehicle, I speed up about 5 MPH and then coast in neutral on the downgrade. In the Fiesta I can get over 50 MPG without affecting traffic flow in any way, except to get the impatient Subdivision driver off my rear end. Maybe I should just let them stay there since they are blocking my low pressure wake and improving my mileage even more. Probably could get past 55 MPG in a car rated at 38 and that's at 65 MPH average speed.

regards
Mech

Question is, will the fuel savings pay for a new windscreen and front end respray?

I see what you are saying, but here on the [wet] coast, the roads seem to stay clean. It is quite rare to be hit by any road debris from anyone. As a matter of fact, I can't remember the last time it happened to me at all.

3 stripes @ 42 feet per stripe separation. 126 feet at least here is more spearation than most traffic averages, at least where I live. Never repainted a front end. The Fiesta has no significant damage to the front end or windshield after 20k miles. Last time I had a broken windshield it was from a car flying by me at 80 MPH. Does that mean I should go 90? I even had a car toss a half pound rock into a customer's windshield at 55 PMH COMING FROM THE OPPOSITE DIRECTION!

I've dodged falling trees, wheels than came off a car coming in the opposite, direction, the remains of a connecting rod from a blown Jeep engine the first time I drove an 89 Maxima home, a tailgate from a farm use vehicle that weighed almost as much as my car, at night.

regards
Mech

Hey now.. That is not in the spirit of hyper-mileing. It is not if the mods (physical and habitual) will save money over time. It is if you can get a new personal best fuel economy stat to put up and show off. ;)

I did my best trip this last Friday. Drafting about 70 miles of it. 102.4 mpg over 119 miles. Let's just say I'm an advocate.

One way to test the benefit of the draft would be so do the same drive at the same speed...that is, IF you usually drive faster than you were that day. Some of that 11% might have been from the slower speed, if you usually drive faster. Not perfect science, but it is still decent data.

The leg home was actually a 30% fuel savings (5.2L/100km vs the normal 7.4L/100km) and this was changing only the highway portion of the leg (47% of the leg's distance). For me that is pretty conclusive - more than the [likely] result of simply lowering my speed.

My normal 60 km/h cruising speed is around 5.5-6.0L/100km, not that I have ever really tested it properly. Maybe I should. Add that to the list.

I know this isn't scientific testing, but I rarely drive this car because it's my wife's. Scientific testing would require burning more fuel than I normally would, and I would rather save the money!

Get behind a big truck while riding a motorcycle. You'll see soon enough how much grit gets thrown at you, even on supposedly clean roads. I used to drive behind big trucks but I avoid them now. Maybe driving a beater it would be OK.

At 2 seconds distance there is no grit strike to speak of.
I see no benefit in getting any closer; turbulence increases close by and ruins the aerodynamic flow, nullifying any gain.
In my biking days I have felt the buffeting of the tail winds in the few instances I was that close by a truck. Usually when merging into a busy highway, the gaps don't get that big...

I can hear the occasional grit stone hit my car, but when that happens there is always someone very close by in the next lane.
It never happens when I'm just alone behind the semi and there's no car in the next lane.

I think you benefit from 3 factors:
1, you are going slower which may help your engine into a more efficient operating speed.
2, the big truck punches a big hole in the air for you reducing how hard your car has to work to go down the road.
3, drivers of big trucks sit up high and can see further down the road and they tend to maintain a smoother speed.

#3 especially! I drive behind trucks because at a 2-3 second distance, it is usually safer. AND of course better MPG...

Following is much safer than leading out alone.
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http://ecomodder.com/forum/showthrea...ing-20209.html
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More about the safety benefits of following was discussed here:
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http://ecomodder.com/forum/378722-post34.html
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3 stripes behind a big rig is right at 130 feet, a little less than two seconds depending on your speed. After almost 24 k miles with a good bit of that in traffic so heavy you get cut off if you leave more space (average separation 3-4 car lengths or 60 feet), my Fiesta's front end still looks great as well as the windshield and it was wrecked and repainted 24k miles ago.

I think I'm #38 on the gas car list and thats an auto tranny. My average speed is close to 40 MPH or higher on almost every tank.

I could P&G engine on and hit 65 if I wanted to average 25 MPH. I generally don't draft when traffic is light, mostly when it is unavoidable, which here is most of the time, and as was posted previously big rig drivers are professionals, expecially thoseewho are employees of corporations like WalMart or UPS. Another advantage is they are "agressive driver repellants".

regards
Mech

We drove 3.5 miles (each way) to the Mexican restaurant last weekend and I got to 50 MPG on that round trip (indicated),from a cold start and a restart.

regards
Mech

Awesome, just loving the warmer temps & summer gas. Jealous of you all down in Tidewater with some of the cheapest gas in the US :thumbup:

I definitely get rocks thrown at me by semis even at 2 seconds following distance, but I do employ Old Mechanic's "pulse draft" method and I try to draft vans and smaller trucks, as they don't rip the road up and spray it at my car. I love when impatient people tailgate me, the drag reduction is really noticable!