How long/far to coast?

[ScottC33]

So let's assume you're the only guy on the freeway, and you have the ability to coast safely from 60 mph all the way down to 20 mph (just as an example), then speed up slowly to 60 mph again (and repeat)? Would that produce the highest mpg, all other things being equal? Or is there some science behind the theory that you should coast only until you hit some predetermined mph before accelerating again?

Obviously, the more you coast (and slower you travel) in neutral, the more fuel you will use to speed up again. What are the current thoughts regarding how far/long you should IDEALLY coast before speeding up again?

I'm trying to figure you whether people coast until hitting 50-55 mph ONLY BECAUSE it's unsafe to go any slower on the freeway, or because there's some other fuel-efficiency issue involved.

This is not an EOC post. All coasting, for purposes of this question, will be done in neutral.

Thanks so much for your input.

Scott

[MetroMPG]

There's no stock answer, unfortunately. It depends (TM).

Some vehicles are so light, or have such poor aerodynamics, or are underpowered enough, that accelerating to /coasting down from highway speeds isn't worth it.

Low speed glides are where the maximum savings are to be had.

Also FYI: the benefit of the pulse typically isn't found with slow acceleration. To take advantage of a typical engine's most efficienct BSFC area, you would accelerate at a moderate/high load, and shift at a relatively low RPM (this assumes you can control RPM because you're driving a stick).

[ScottC33]

Thanks for the reply. I actually have a gas-guzzling Tundra, and I drive at off-hours. I am squeezing 21 MPG out of her (combined), but I'm looking for more....just want to coast as long as I reasonably can if there's very little traffic around (assuming it's the right thing to do, MPG-wise).

And I have an automatic. It does have the trip-tonic feature (ability to shift myself), but I'm not sure if I should use that in lieu of the auto transmission.

Thanks again.

[MetroMPG]

The fact that it's an auto & has poor aerodynamics suggests to me anyway that you wouldn't see any gains, and possibly a loss, by doing engine-on pulse & glide at highway speeds.

Of course the only way to know is try it - if you've got fuel consumption instrumentation.

[ScottC33]

So if you don't mind my hijacking my own thread and asking you this.........

What would you suggest is the best hypermiling technique for a vehicle as heavy and aerodynamically retarded as my truck? I've read hundreds of threads/articles/etc., but they almost exclusively pertain to much smaller vehicles with manual transmissions and better aerodynamics.

I assume there aren't many full-size truck owners on this site.

[MetroMPG]

I'm assuming you're still asking about highway techniques. If you've read that many threads/articles, then you probably already know the basic prescription for freeway driving:

1) Slow down. (Have you figured out at what constant speed your truck gets its max MPG?)

2) Drive with load (DWL) where possible - you really need instrumentation to do this, either a vacuum gauge or onboard MPG display. See the description of this technique in the hypermiling tips list.

3) Aero mods. Have you seen bondo's aerodynamic F-150 truck cap? He claims an improvement from 18 to 22 mpg on the higway.

There are quite a few mid-size and more than a couple of full-size pickup owners on the site. Hopefully others will chime in with tips/encouragement.

[greenitup]

i drive a ford ranger, it is alot smaller but has the same type of aerodynamics and weight for size. I have found that slowing down alot saves money, it bumps me us a few mpg. When i drive on the highway, im always in the right lane and always way under (55, assuming safe) If you go slower than traffic then when the vehicles pass you their waves they create in the air "push" you along saving you money too. Hope you get you mpg higher.

[Z man]

I would only coast on the highway if it is going downhill significantly in places.

Also, slowing down to 20mph just wastes time and keeps the truck running longer if you think about it, a steady 55-60mph works great for me.

[ScottC33]

Thanks for all the input. Best I can tell (based on four of five tanks), the Tundra has max fuel efficiency in the mid-50s (mph). I haven't really tried anything slower than that for fear I'll get shot. :-)

I actually just finished a 30-mile freeway drive. I went about 55 mph (average) and drove behind (but not too close) whatever big trucks I could find. I was up near 30 mpg for those stretches when I followed a big truck. I was at about 24 mpg for the non-drafting stretches. That seems like an unrealistically high differential to me, but who knows?

Since uphill/downhill and wind may have been in my favor (but I don't think so), I'll keep experimenting with it. The aerodynamics on my Tundra may be so God-awful that this drafting technique may yield far better results than anything else.

Happy Monday!!!!

[skyl4rk]

To answer your original question:

<theory>

For pulse and glide, choosing the speed to engage your gear (initial velocity) should be based on what speed your transmission will go into gear at a low rpm. In other words, experiment to find out what speed you can engage the transmission and have the motor turning at relatively low rpms in acceleration. This starting point may be one or two gears down from your terminal velocity, you may have more than one initial velocity.

To choose the speed to go into neutral (terminal velocity), it should be at a moderate rpm in your top gear. Also always consider that you want your terminal velocity to be relatively low to minimize wind drag.

Coasting is most effective when you accelerate or maintain speed up hills and glide down them.

<\theory>

Driving our Chrysler minivan, I see gains in highway mpg by 1. driving at a slow speed (55mph if possible) using cruise control and !!! 2. long coasting to off ramps !!!. Sometimes I can see the total trip mileage over a trip of an hour tick up several tenths of a mile in the final approach to the exit ramp.