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Old 07-18-2009, 10:49 PM   #1 (permalink)
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Speed vs. Semi

JACK MY OWN THREAD******OK I have an important question for everyone, will P&G give me better MPG if I'm in an auto that cant be flat towed, so I would basically be going up to say_ 60 mph, then putting it in neutral, going down to 50, putting it in drive, (repeat) since I would not be turning off the engine and all that mess? I have been wondering for a long time******



Ok so if it is night I will usually go 50 on the highway, the other night a semi passed doing 65-70 mph, I got behind him (going almost 70) traveling about say...80 feet behind him, which would be better for MPG, just going 50, or going 70 behind him?

PS: Please don't gave a great debate about it being "dangerous" or whatever, Iv seen people travel behind semi's at like 15 feet, they are pretty crazy...


PSS: How old is everyone here? Just wondering, Im 19, turning 20 on July 25th


Last edited by RandomFact314; 07-20-2009 at 06:15 PM..
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Old 07-18-2009, 10:59 PM   #2 (permalink)
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Probably 50 MPH, since the lack of load at higher RPM's following the tractor is putting you outside the good area for BSFC. (Sure, you're using less fuel than you would at 70 normally, but you're still revving quite a bit higher as well. I think the higher revs outweigh the aero reduction.) Speculatively, of course.

Yeah, it's dangerous. It's also quite common these days.

24, 25 next Feb.
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Old 07-18-2009, 11:27 PM   #3 (permalink)
My Goal: 35 MPG All Day
 
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Quote:
Originally Posted by Christ View Post
Probably 50 MPH, since the lack of load at higher RPM's following the tractor is putting you outside the good area for BSFC. (Sure, you're using less fuel than you would at 70 normally, but you're still revving quite a bit higher as well. I think the higher revs outweigh the aero reduction.) Speculatively, of course.

Yeah, it's dangerous. It's also quite common these days.

24, 25 next Feb.
I'm not sure what BSFC is, I know what it stands for but dont understand it... But I DO get that you are going with the 50mph on cruise control rather than the 70 without cruise control and having to fluctuate b/w 65 and 70 is bad.
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Old 07-18-2009, 11:30 PM   #4 (permalink)
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Brake Specific Fuel Consumption - Literally, the number of grams of fuel required to produce 1 Horsepower Hour of work from your engine.

Your peak BSFC occurs when you're producing the most torque for the least fuel. It's not always the most economical place to drive, but it's the most efficient. (If your BSFC island falls in an area that is making more torque than you need to operate at speed, it's not most efficient to downshift so that you're in that range. Instead, upshift and use lower RPMS, which will indefinitely use less fuel.)

BSFC (IIRC) is based on WOT, and sea level, though.
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Old 07-19-2009, 11:24 AM   #5 (permalink)
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> BSFC (IIRC) is based on WOT, and sea level, though.

I believe you indeed recall correctly, at least as far as available data goes. I've found it terribly hard to get BSFC info for anything other than full power at a given RPM. I'd expect BSFC to get worse as throttle (or fuel pump setting for us diesel folks) is reduced, since at low throttle there's still the same base internal engine friction to overcome as at high throttle (the extreme being whatever fuel is needed to idle--there's fuel going in but no power coming out) but how steep that curve is would be valuable to know.

It's been my experience that the rpm for optimum vehicle efficiency is lower than the rpm for optimum engine efficiency. In cruise, there's always more power available than is being used (otherwise we could never accelerate to cruise speed, plus we'd not have any excess available for when we needed it for hills or passing or whatever) and at some point there's a "sweet spot" where the less-efficient-due-to-low-revs curve crosses the less-efficient-due-to-low-throttle curve.

[edit PS--I turned 61 in June, but prefer to think of myself as 16 Celsius]

Last edited by JackMcCornack; 07-19-2009 at 12:06 PM.. Reason: response to question
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Old 07-19-2009, 12:34 PM   #6 (permalink)
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Quote:
Originally Posted by JackMcCornack View Post
> BSFC (IIRC) is based on WOT, and sea level, though.

I believe you indeed recall correctly, at least as far as available data goes. I've found it terribly hard to get BSFC info for anything other than full power at a given RPM. I'd expect BSFC to get worse as throttle (or fuel pump setting for us diesel folks) is reduced, since at low throttle there's still the same base internal engine friction to overcome as at high throttle (the extreme being whatever fuel is needed to idle--there's fuel going in but no power coming out) but how steep that curve is would be valuable to know.

It's been my experience that the rpm for optimum vehicle efficiency is lower than the rpm for optimum engine efficiency. In cruise, there's always more power available than is being used (otherwise we could never accelerate to cruise speed, plus we'd not have any excess available for when we needed it for hills or passing or whatever) and at some point there's a "sweet spot" where the less-efficient-due-to-low-revs curve crosses the less-efficient-due-to-low-throttle curve.

[edit PS--I turned 61 in June, but prefer to think of myself as 16 Celsius]
That's funny. I'd be -4.444. Good Bye Bills!
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Old 07-19-2009, 02:21 PM   #7 (permalink)
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Quote:
Originally Posted by RandomFact314 View Post
Ok so if it is night I will usually go 50 on the highway, the other night a semi passed doing 65-70 mph, I got behind him (going almost 70) traveling about say...80 feet behind him, which would be better for MPG, just going 50, or going 70 behind him?

PS: Please don't gave a great debate about it being "dangerous" or whatever, Iv seen people travel behind semi's at like 15 feet, they are pretty crazy...

PSS: How old is everyone here? Just wondering, Im 19, turning 20 on July 25th
I ran some rough numbers for my car, a Land Rover LR3 HSE (what else would a hypermiler drive?) and wrote a blog post here. Driving 55 m.p.h. unaided and 59 m.p.h. drafting at about 50 feet should produce about the same fuel economy. Drafting at anything faster would decrease fuel economy from 55 m.p.h. unaided. Needless to say, not too many trucks are going slowly enough to make it worth it for me.

I'm not willing to get closer than 50 feet due to some calculations I made that are detailed here.

Last edited by PA32R; 07-19-2009 at 02:25 PM.. Reason: Add second paragraph
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Old 07-19-2009, 03:45 PM   #8 (permalink)
My Goal: 35 MPG All Day
 
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Yeah I drive 55 mph on the highway in the day and 50 at night
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Old 07-20-2009, 12:29 PM   #9 (permalink)
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I think you would have more gains to just go 50. I think trying to maintain speed based on another vehicle wouldn't be as good as just letting the terrain and load guide you with your speed. I for one can't stand being behind a semi, I rarely notice any gain, and also the truck's wake rips and twists at me and drives me absolutely up the wall, I can't wait to get out of it. Thankfully most of them are going faster than me.

Oh by the way, I will be 33 on July 25. Happy early bday!
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Old 07-20-2009, 01:04 PM   #10 (permalink)
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Ok, so I'm going to be the rebel, Seeing as I just got my best trip ever headed down to Van Meter from Ames, about 60miles one way the other day. I was in a bit of a hurry so I was doing 70's the whole way. Seems there we're a lot of pickups with trailers headed the same way, so I did some hoping from one to the next as they passed. Following about 6-7 car lengths behind. Managed to get 26.9MPG's on the way down. My way back on the other hand, it was much colder (65F as opposed to 79F on the way down)and there was literally no traffic at all. The best I could push out of it was 23.2MPG's.

In my case I would imagine the reason for it being so much better is the extremely poor aero on the Jeep Cherokee. I believe cd is 0.51, so having the trailers push the air around the big hog helps more then a small aero dynamic car would.

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