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If I want to get the very best mileage on battery when on he highway, is there an official "most aerodynamic/efficient" speed to stay at?
 

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I don't think we have a solid answer for the Volt. With most cars, the most efficient speed is the slowest you can sustain in the top gear. Tesla concluded that the most efficient speed to drive the Roadster at is 17 mph - which seems to be a fairly common range for electrics.

The Volt changes strategies right around 62. Below this it spends a lot of time in virtual 6th gear - MG B (the larger main electric motor) more or less stopped, with the engine driving the car directly at 3.1:1 through the planetary system (thus virtual 6th gear - it's in the same range as most car's 6th gear) and whatever excess engine power there is (always at WOT, remember) pulled out on MG A and sent to the battery. When it builds up a few hundred watt hours, it shuts the engine down and runs electrically for a mile or two.

Above this range, it'll run the engine continuously, at lower RPMs (it never runs slower than ~1400 RPM) in a Prius style CVT relationship. I'm not 100% certain, but I think the reason for this change is that at 1400 RPM it is generating too much power for the steady state road loads below 62 mph.

My original assumption was that the most efficient speed in CS would be the slowest that the car goes parallel, since series is 10-15% less efficient. This may turn out to be the case, but it turns out this isn't "highway speed" - it's about 35 mph.

Also note that because it's all power traded on the high voltage bus, both A/C and especially Heat have an outsized impact on economy (even after the engine starts, the car will run the electric heater under some circumstances if it's set to comfort.)

So do a little experimenting, but my overall advice is the best economy is as slow as you are comfortable going...
 

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Sorry to tell you this, because it's probably not an answer you want, but, as Walter says, the answer is very simple: The slower the better. The Volt will use much less energy per mile at 55 than at 60 because the power needed to overcome the drag force goes up by the cube of the velocity. Switching to both motors will occur whenever this is more efficient, and it may be more efficient at a lower speed if your speed is constant, but you'll use less energy at a lower speed regardless of the how the drive train is configured.
 

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Seems like I remember reading somewhere that the farther away from 30 you are the worse. Not sure if that was for in town speeds though
 

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The question was what highway speed maximizes electric range. The most accurate answer is the minimum highway speed allowed or that you are most comfortable with.
 

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I have been pretty content setting cruise around 56-57 and hanging out in the right lane watching all the SUV's going 80 in the fast lane and watching the $5 dollar bills coming out of their tail pipes! lol
 

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43.5 mph and a tailwind does help. That's most efficient and may give you 55-60 miles on battery on a flat highway in Kansas driving eastwardly. And, there is always drafting off a Prius. :)

Or, drive it like it's a regular car and have fun with it. Mileage is affected more by ambient temperature and tire pressure than trying to be efficient at 55 or 60 or even 65. Ambient temps affect whether you use climate controls in the car too which affects range. 70*F and cloudy with low humidity should offer the best "environment" for highest mileage. All this really does is make you way too detail oriented to really enjoy electric driving. I wish (I really do) people would want to make their ICE vehicles higher mpg with choosing an efficient highway speed or tire pressure. I'll run my current car (Mazda6 wagon) at about 63mph on the highway with harder tires and get 2mpg better than the car's EPA rating for highway. But others may go 80mph and not check tire pressure and lose 3-4 mpg in the same model car.
 

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DonC is right...aero drag is the single biggest factor affecting efficiency at "highway speeds". Fluke's data on the other thread may be helpful if you're looking at low speed data (under 50 mph).

Here's the way I look at it. If you can get to where you're going on electricity alone, then drive as fast as you want. Those electrons are cheap. My commute puts me on the ragged edge of the Volt's range, so I've found that I drive slower (70-75 mph) in the first 1/2 of my commute and if if the remaining distance and SOC charge indicate that I can drive harder on the second 1/2, then I do. My goal is to show up at work with less than 5 EV miles remaining...so I can get a nice full (free) charge at work.
 

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Here's why people mention the parasitic load.

When you are driving, your efficiency is based on how much battery power is used. The battery power used is primarily based on propulsion, but everything else in the car that uses the battery affects your efficiency. For propulsion, you might use perhaps 5,000-20,000W on average. In general the faster you go, the less efficient you are. *But*, the faster you go, the less time the car is on.

So if you have, say, 500W of a parasitic load (headlights, windshield wipers, radio, etc.), and drive for 1 hour, that would be .5kWh used. If you double your speed, you only drive for 30 minutes, and only use .25kWh. So even though you drive at a much less efficient speed, you gain a bit back.

During winter, this really comes into play, allowing faster speeds to be more efficient. At ECO full blast (which I find I often have to do for an entire trip in the winter), it's somewhere around 3kW. Driving 20 miles at 60MPH takes 20 minutes, but driving the same distance at 70MPH takes about 17 minutes. That 10MPH change in speed reduces your efficiency, but that 3 minutes saved .15kW (about 1/2 mile of range).

Looking at the numbers also helps show that listening to music and having the headlights on barely affects the range. While a 55W headlight bulb seems like a lot, when you compare it to the 5,000-20,000W or so of power to drive the car, it's a drop in the bucket.
 

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If it's cold with four people in the car with snowy clothes/shoes that are melting and you have the climate settings on comfort and 76 degrees with the defog and defrost on, the climate system can use more than 3kW. I don't know what the exact number is, but one Volt advisor who posts here mentioned "more than 7kW". Even with "fan only" in cold temperatures I never saw more than ~33m range, so I think there are other parasitic loads in cold weather. Those loads have an impact on optimum speed for range, probably pushing it up into the 45 to 50 MPH range.

Now that it's warm, my low-speed commute is looking much better. I saw 4.5 m/kWh today. The same trip in winter, even with "fan only", maxed at about 3.3 m/kWh. Parasitic load is important. Drive faster in winter, slow down in summer. :)
 

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For many years I used the instantaneous MPG display on my VW diesel Jetta. Although the absolute number shown may not be 100% accurate, the delta when drafting or facing a headwind was important. On long highway trips, facing a headwind, I could see a 30-40 MPG increase by "drafting" another vehicle. Many times (if I was not in a hurry), I would pull in behind a semi-trailer that was driving say 5 MPH slower than my speed. Depending on wind and terrian, the following distance can be safe and still benefit greatly from the draft.
I still use these techniques today when driving the Volt, especially if I know I will be pushing the battery range. Drafting (safely) can dramatically reduce the amount of air the car needs to push to maintain the typical 60 to 70 MPH crusing speed on our highways.
 

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If it's cold with four people in the car with snowy clothes/shoes that are melting and you have the climate settings on comfort and 76 degrees with the defog and defrost on, the climate system can use more than 3kW. I don't know what the exact number is, but one Volt advisor who posts here mentioned "more than 7kW". Even with "fan only" in cold temperatures I never saw more than ~33m range, so I think there are other parasitic loads in cold weather. Those loads have an impact on optimum speed for range, probably pushing it up into the 45 to 50 MPH range.
The electric heater wide open (only permitted by the comfort setting) can draw 6.5kW. In addition, the fan can draw up to 200W or so IIRC, and under some circumstances (high relative humidity,) the car will run the A/C in parallel with the heater to defrost. I'm not positive of the max A/C draw - it's somewhere in the 3kW neighborhood.

Yes, there are certainly other factors in winter, though mostly not parasitic loads. The 400W of rear window defroster and ~40W seat heaters are parasitic loads. The rest... Tires don't roll as easily, transmissions don't turn as easily (cold temps lead to higher viscosity in all the lubricants, and so more friction to overcome.) Batteries actually have less physical capacity (which the car knows - in the winter it was usually "dead" at 9.6 kWh used on the display. In the late fall I was seeing 10.2 kWh, which I expect to see again soon.)

The classic rule of thumb was that the most efficient speed is when the fixed/time loads and the speed based loads were equal. With no climate control, that's something really low - the 400W of per time loads probably matches something around 15 mph (60 mph on ~12 kW? = 30 mph on 3 kW = 15 mph on 750W? Lots of indefinite assumptions in this math, folks, so don't take it as gospel - I just don't have a total load model. This assumes most load is drag related - which is true above 60mph usually, but increasingly less valid below that.)

With the heater running full power, the most efficient speed likely climbs into the 45 mph range. Note that the car will still get less range at 45 mph in the winter than it would at 45 in the summer, and will get less range at 45 in the summer than it would at 20 in the summer - it's just that with the parasitic heater load, driving at 20 in the winter will get still less range because the heater will eat the lion's share of your battery instead. Make sense?
 
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