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

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.

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.

> 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!:)

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.

Yeah I drive 55 mph on the highway in the day and 50 at night

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!

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.

Happy early birthday to you too :) We both got 5 more days :)

Oh wow, I never even thought about the fact that worst cds would get more of an advantage but I see that obviously makes tons of since.... You taught me something today (or at least made me realized the should-be-obvious) :D

I tested this once. 70 mph at a 2-second following distance was about the same as 65 driving solo. 60 was even better, and 55 and 50 even better still.

Drive solo. Don't draft. Besides the safety aspect, it doesn't gain you THAT much.

Yeah I see that it probably will not do me much good so ill just stick with going 55 day and 50 night :thumbup:

Im going to be the voice of opposition based on my experiences. My best ever trip MPG (54MPG) was drafting a truck and that was with the A/C on the whole way. The truck was doing under 60MPH the whole time and I was doing pulse and glide between about 55 and 60. I draft whenever possible and it always seems to make a difference in the positive and a fairly significant one at that.

I've done 55 mpg with P&G behind a truck at about 60 mph, but my best highway trip was 71 mpg P&G with no truck. Trying to keep up with a truck really handicaps you and keeps you from making the most of the P&G rhythm.

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...

As long as the engine is running you should be OK to go in and out of gear since the pump that lubes the tranny will still be running. The issue tends to be when EOC is attempted in an auto.

It's possible, but much harder, with an auto. I find the best results with an auto come from keeping steady speed, losing a bit on the uphills and gaining on the downhills, and some strategic engine-on coasting. Find a good speed that gives you just enough power to stay in top gear without downshifting, and don't go much faster than that.

No the question is will it help my MPG still?

So if I'm going down the highway at 50 at night, do you recommend I just keep on going 50 with cruise control or should I do P&G my way? P&G @ 60,neutral,50,drive (Repeat)?

The mpg benefit you get from drafting is definitely related to the Cd of your vehicle. The Cd on my car is so low I get almost no benefit from drafting.

The mpg you get while drafting is a quick and dirty way to estimate your car's aeromodding potential since the only thing that changes while drafting is the airspeed (and hence the wind load) experienced by your vehicle. Engine rpm's and rolling loads remain the same.

As far as shifting in and out with the automatic, it depends on the vehicle. Mine for one does not like to go back into gear at anything above 25 mph. Anything above that it pretty much has to run itself through the gears again and then takes a bit for the TCC to re-engage. So it's not worth it to P&G for me. Even trying to lose speed, then regain speed.. doesn't amount to any gains either. Your mileage may vary.

Some autos are quick to release the transmission and go into a semi-neutral mode when you let off the gas - GM models in particular. Others, like Hondas, are very strong on holding it in gear, giving you heavy engine braking whenever you let off the gas. Clearly, in the 2nd case it's more advantageous to shift to N, while in the first case you really don't need to, since it's already doing it for you.

I would hold speed at 50-55 mph. Cruise control is good on flat ground, but as hills come in, you'll do better with manual control. CC is a reactionary device, whereas you can anticipate. CC will overreact and floor the gas, causing a downshift. In the same locations, you can hold it in top gear yourself.

Yes, P&G still works in an auto that you can't EOC in. 60 to 50 might not necessarily be the best choice of speeds, though.

BTW - still 24.

Oh - I didn't really document it, but I did some P&G in Granny for awhile... didn't really amount to anything, but that was varying speeds from 60-45, 65-45, 55-40, etc... trying different things, but I didn't get any real results that I could safely attribute to anything. In Granny, it was better to just leave her in gear, since the tranny basically goes into neutral on it's own.

Yeah Crist I know it does not alert the subscribers when I do things like that, I just would not like to make thread after thread after thread and seem like a d-bag to answer simple questions. Like in some of my threads I have asked one thing, and when its answered iv asked about how much change will I get removing my antenna, which I don't think people would like if I made a thread over something so easy to answer...

No, what I was suggesting is that you ask the question later in the thread, but still edit your original post if you please. Asking later in the thread will bump it up the list for a new reply, and that will alert people to check it.