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Old 07-01-2008, 07:27 PM   #1
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Question regarding drafting

I just did a 600-mile roundtrip from Vancouver, BC to Portland, OR. Since my normal driving consists of 100% city driving here in Vancouver, it's nice to see what my car can do once in a while on the freeway.

I got 41mpg in my auto 98' Corolla. Quite pleased with that. I did a similar trip last year and got 37mpg. The only difference this time is a partial lower grill block, and 35psi max sidewall instead of the recommended 30psi.

It was the hottest weekend of the year (100+degrees F) and we had to blast the A/C on recirculating 90% of the time. I was a bit worried about the grill block, but the temperature needle stayed at the halfway point as usual.

I tried to stay 3 seconds behind big rigs whenever I could, but the scenery gets really boring. My question regarding drafting is this: when faced with the choice of 65mph behind a big rig, or 55mph open air, which is better? I don't have a Scangauge and I realize the results are probably different for different cars. But as a general rule, what should one do in this real world situation?

Attached image of my grill block. Cardboard and black tape.
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Old 07-01-2008, 10:07 PM   #2
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Safety first! Make sure you have enough room to stop...try also to have a little vision around the truck. If you are concerned, then it's probably better to drive 55mph and see rather than 65mph blind.

I've been known to draft, however I like to be able to see around the truck (and I usually only draft on lightly travelled roads). Might use a little more gas that way rather than being next to the truck in the suck zone, but lot of good that would do me if the truck hit someone in front of him and I wound up jammed under the trailer...
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Old 07-01-2008, 10:28 PM   #3
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I find that going alone is better than drafting when the trucker's driving style requires me to stomp on the gas to keep up and stomp on the brake to stay back. If the trucker's driving style allows me to keep a light throttle and no braking to keep my distance then drafting works very well.
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Old 07-02-2008, 07:03 AM   #4
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3 seconds is a good safe distance for drafting. I don't go closer than that. Sometimes they cut in front of me much closer for a few seconds, but I don't seek it out.

55 solo would be the best
60 solo is about the same as 65 drafting, in my experience
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Old 07-02-2008, 10:15 AM   #5
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It would seem you always get a lot of aero benefit in drafting, however, your ability to make use of it is hampered by having to keep the engine turning at higher revs to maintain speed against the rolling resistance and the last 20% of the aero drag it doesn't reduce. In other words, you might be burning more gas than necessary to keep moving, just from maintaining RPM. If you had an infinitely variable ratio gearbox, you could take advantage of any speed draft, but at very low power requirements at high revs, your motor is probably using more gas fighting the throttle plate (pumping loss) than it's using to keep you moving. Round about 3000 RPM is to motor frictional and pumping losses what 60mph is to aerodynamics, the drag is a square law relationship that goes vertical around there, so if you can draft at a speed requiring less than 3000 revs you're probably saving gas, if you have to go over 3000 revs, you may not be.

I guess if you had a small truck, you could get an overdrive fitted, purely for drafting, and likely get very odd looks from the guy who fits it when he tells you your puny 4cyl motor won't be able to keep you at 60mph when you engage overdrive, and you just nod and smile. That would be because you realise you only need 10HP to draft at 75mph and 1800 RPM instead of the 30HP and 3600 rpm you normally need at 75mph, but just keeping the motor turning at 3600 may use as much gas as needing 25HP at 60mph.
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Old 07-02-2008, 10:19 AM   #6
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Quote:
Originally Posted by zimagold View Post
65mph behind a big rig, or 55mph open air, which is better?
Someone asked exactly the same question (but the speeds were 65 vs. 70, IIRC), and sparked a long and informative thread. I linked that thread in my sig, click on the bit about drafting.

IMO, it's probably better for FE to draft, but the cutoff point for when it's better to draft vs. when it's better to slow would be different for each car.

Your grille block looks great.
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Old 07-02-2008, 10:20 AM   #7
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Safety is always first. I think 3 seconds at 60mph is about 260 something feet of spacing - plenty of time for reaction. In fact, I had a piece of drywall come hurtling towards me on this trip and had enough time to steer safely around it. Hypermiling makes you concentrate on the road way harder than if you were casually cruising down the freeway.

The only hard numbers I've seen is from the Mythbusters episode where they noted an increase at 150 feet back. That's way too close. I really should just get a Scangauge and figure the numbers out myself, but I was hoping someone had already done the work. If the difference is negligible at 3 seconds, I'll just stay in the far right at 55-60mph.

The other concern is tire separation from a big rig. What usually happens in that situation? I would imagine the most dangerous spot is right beside the big rig - that's why if I'm passing one, I'll do it as fast as I can with no regard to fuel economy. I think the wheel will separate to either side until it hits something, at which point, it can be deflected in any direction. My theory is that directly behind the big rig is actually one of the safer places in the event of a tire separation.

@theholycow: Oops, somehow I missed that long thread. Thanks for pointing it out and sorry for starting this thread.
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Old 07-02-2008, 10:30 AM   #8
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Yeah I've seen tires blow out on semis and coaches and the worst place to be would be in the lane alongside.
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