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Old 11-12-2012, 04:39 PM   #1 (permalink)
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Question The big bad motorway vs the tiny car

A bit of advice from the assembled "expertness" if you please

I was trundling with George along the M9 motorway (aka freeway) last week. Normally I like to trundle along in the left hand (i.e. slow here) lane with the trucks so that a) I can get some limited draft effect (I don't tailgate) and b) so that people coming up behind apparently blind whilst driving at 85+ (limit is 70 but hey...) notice me and I don't get those last minute panic lane changes.

But on this occasion there were few trucks so I had to scurry along about 5-10 MPH faster than normal. That combined with changing wind direction (its a hilly area and it was stormy) and unfriendly inclines meant my MPG was low compared with recent averages, about 55-60 vs 70+ (imp).

I tried P&G but pulsing at these speeds (60+) is kind of limited before you have to do it again and George is just not that aero and is very light - the net result is falling MPG not steady - and of course I managed to pulse just before we hit another headwind of incline - Not sure if anyone has seen the old cartoon of Homer Simpson falling down a cliff and hitting everything on the way but it kind of felt like the ecomodder equivalent of that whien I tried stuff.

Eventually I decided on a DWL cruise with 60 as the minimum speed and anything over 55 as the MPG target, but ended up with a low-ball average of 56 for the round trip - not good.

So a question for the Metro-style folks - how to trundle at higher speeds (say 60-65) in the most efficient way in mixed conditions on an empty road except for speed freaks.

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Old 11-12-2012, 06:13 PM   #2 (permalink)
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P&G will always be the most effective driving method. However, if you're going into a head wind and are already at a decent speed the technique might be nearly moot. The purpose of P&G is to only operate the engine at its most efficient level - highly loaded. If you are already achieving a fairly high engine load because of the wind and speed you don't really need to glide as the engine will continually be loaded an adequate amount. I suggest watching the LOD gauge on the scangauge to determine when this is happening. By looking at a few BSFC maps, it seems anything over about 60% load is putting you pretty close to ideal BSFC and therefore the benefit of P&G is drastically reduced.
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Old 11-12-2012, 07:59 PM   #3 (permalink)
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P&G doesn't work as well at high speeds in low-powered, unaerodynamic cars. The pulse is too long & the glide is too short.

As Daox points out, the headwind only makes things worse.

DWL or "target driving" to a reasonable (under the circumstances) MPG figure is the next best approach.
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Old 11-13-2012, 04:40 AM   #4 (permalink)
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Gave DWL a better go on the comute this morning - will have to see how it hacks it on a longer trip with higher speeds. P&G (I still engine on most of the time) works well under 55.
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Old 11-14-2012, 10:27 AM   #5 (permalink)
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Terrain dictates, Mr. A, and on that road in your car the best you can hope for is for your mileage to not suck too badly. Look on the bright side- your worst is about twice as good as one of my bad days!
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Old 11-14-2012, 04:07 PM   #6 (permalink)
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Terrain dictates, Mr. A, and on that road in your car the best you can hope for is for your mileage to not suck too badly. Look on the bright side- your worst is about twice as good as one of my bad days!
Yep - it's more an average thing, I could almost feel the pound notes being extracted from my wallet as petrol is $8+ US Gallon here.

I wish I could have gone slower but the 80+ muppets (I used to be one) don't notice anything small and slow.

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