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Old 08-01-2008, 06:59 PM   #1 (permalink)
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Drive Slower or Drive Faster behind a Semi? Which is more efficient?

Hey all I have a question about which technique would be better. I'm new here and just so you guys know I have a Dodge Ram V-8 so obviously i'm obsessed with hypermiling lol.So here it is

I do most of my driving on the highway and to save gas I drive around 60-62 MPH and my tach is about at 1,400 to 1,500 RPMS. I have heard that drafting behind a semi will significantly increase gas mileage by about 30%. My problem is all the semis drive about 70-75 MPH and in order for me to draft successfully behind them I have to maintain that speed and my tach goes up to about 1,800 to 2,000 Rpms! I don't yet have a Scan gauge so I cant really tell if i'm really getting better mileage because I cant exactly stay behind a semi every time I drive but when I draft behind a semi the whole way should I see my RPMs go down a bit because of the great decrease in wind resistance?
I just want to know that would I be saving more gas driving at a slower speed than driving at a faster speed behind a Semi? Thanks all

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Old 08-01-2008, 07:13 PM   #2 (permalink)
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drive in front of it. Your like the cow plow on an old steam train for air flow, and the rig will push you along while saving fuel himself. (seriously)
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Old 08-01-2008, 07:20 PM   #3 (permalink)
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I would drive at the most efficient speed for that truck, regardless of the semi's and if possible, stick to the slow lane. Avoid fast followers and follow someone at your speed and you should be ahead of the MPG curve.

The problem in trying to keep up with a fast semi is that you might be using more gas to accelerate/maintain that speed than you will save on the drag reduction. Without a scangauge you won't really be able to tell for sure if you are making any savings until your next refill.

Also comes the question of drafting -- how close/far would you have to be to get any real benefit at those speeds? If it means moving to "death" range (ie 0-2 seconds behind them) the risk of injury/damage is not worth it plus it is illegal in some jurisdictions.

Figure out your truck's fuel efficiency sweet spot (best speed @ lowest rpm in highest gear) and work around that -- at least that's what I would do.
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Old 08-01-2008, 07:23 PM   #4 (permalink)
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First: get a ScanGauge. It'll pay for itself many times over if you're serious about saving fuel. And with a thirsty vehicle like yours, the payback will be fast.

As for drafting: just chill out and drive slower, if traffic permits. I'd never recommend drafting, mainly for safety reasons. But in addition to that, the rigs never keep a constant speed, and you'll find you have to hunting your speed up & down to stay with one.

On top of that, another good highway technique is "driving with load" (DWL) on inclines/descents. Anything you draft will almost certainly not DWL in those situations, so even in a draft, you're giving away potential fuel savings there.
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Old 08-01-2008, 07:36 PM   #5 (permalink)
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Here is what I do. I will draft a vehicle (not close-in) and stay with it as long as it suits my agenda. When he pulls off and leaves me, no problem, there is always another vehicle coming along that I can draft.Try to maintain the speed you want to go, don't let others determine that for you. I even make a game of it. I love to draft gasoline trucks and Hummers.
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Old 08-01-2008, 10:35 PM   #6 (permalink)
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Quote:
Originally Posted by bgd73 View Post
drive in front of it. Your like the cow plow on an old steam train for air flow, and the rig will push you along while saving fuel himself. (seriously)
In NASCAR, a drafting pair is slightly faster, but only the second car saves fuel. I'd also get on the CB for permission before even trying that. (if that advice was serious, so was Jerry Lewis. This one is Serious as Cancer)
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Old 08-01-2008, 10:37 PM   #7 (permalink)
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Quote:
Originally Posted by azraelswrd View Post
The problem in trying to keep up with a fast semi is that you might be using more gas to accelerate/maintain that speed than you will save on the drag reduction. Without a scangauge you won't really be able to tell for sure if you are making any savings until your next refill.

this is so true I just got my scanguage and noticed right away driving with constant throttle position nets great mpg numbers but trying to speed up and slow down to maintain a speed on local slightly hilly terrain really hurts, I see the same kinda thing trying to stay behind a truck....

bottom line get a scanguage!!!
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Old 08-02-2008, 11:54 AM   #8 (permalink)
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Indeed, a scangauge or MPGuino will be helpful...but don't draft, truckers don't like it and it's not safe.
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Old 08-06-2008, 02:56 PM   #9 (permalink)
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As a fellow Dodge Ram driver I would heartily recommend drafting. In my area I don't like to drive quite as slow as you but typically do 65 mph. Trucks around here typically go the same speed or about the flow of traffic.

I would gladly speed up to 70-75 to draft a truck. When I'm at 75mph my engine is at 2100 rpm which is pretty much it's sweet spot for BSFC. At that speed you can be a full 2 seconds behind a truck with a box trailer and get most of the benefit of the draft. In my area 2+ seconds is another "death zone" because you'll get cut off, so 1-2 seconds is the acceptable following distance (typically at 60-65mph) and drafting works very well. At 1-2 seconds following at 60-65 mph I can draft minivans, other pickups, and CUVs/small SUVs.

I would say most truckers do attempt to DWL to maintain momentum. It would burn too much fuel and require too much shifting for a trucker to maintain constant speed on hills. Maintaining a DWL in the draft of a truck is a little hard in the strict sense, but it's easier to P&G especially since a heavily loaded truck will slow down more on uphills and the draft helps extend your glide.
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Old 08-06-2008, 04:43 PM   #10 (permalink)
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1 second is just too close behind someone. ESPECIALLY if you are driving a pickup truck. Wake up, folks, you can not stop very fast, you can not steer very fast. 2 seconds is the minimum that I would drive my CAR behind anybody at those speeds. How about this strategy for the original poster. Drive 5mph below your desired driving speed and wait for a truck to catch you that is driving your speed and pull in behind. Try that for a while.

I find that trucks generally do maintain very even speed on flats, speed up on downhills and slow down on uphills. You just need to pick the right driver. Funny thing, i never follow the trucks with america plates because they all drive faster than the trucks with canadian plates.

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