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Speed up 10mph, lose 45 miles range

For both the Roadster and the Model S, you lose 45 miles in full-tank flat-road range for every additional 10 mph speedup.

This curve, from my PDF slideshow at http://WilliamCalvin.org/EV, shows the Model S. The ghostly curve just below it is the Roadster with its lower overall range but same slope.

Indeed, wind resistance is a bitch...

http://en.wikipedia.org/wiki/Wind_resistance#Power

The power needed to push an object through a fluid increases as the cube of the velocity. A car cruising on a highway at 50 mph (80 km/h) may require only 10 horsepower (7.5 kW) to overcome air drag, but that same car at 100 mph (160 km/h) requires 80 hp (60 kW). With a doubling of speed the drag (force) quadruples per the formula. Exerting four times the force over a fixed distance produces four times as much work. At twice the speed the work (resulting in displacement over a fixed distance) is done twice as fast. Since power is the rate of doing work, four times the work done in half the time requires eight times the power.

That tells me that on the long trips that I do rarely, I got to stick to 60 miles on cruise control, until Tesla deploys superchargers. Otherwise, I will have very margin for error and may to stop at some slow charger for 2 hours. Might as well enjoy the music with HVAC on.

Like to have smart cruise control.

Yes, speeding is best avoided especially on long trips. Plus slowing down is safer, anyways: accident a bit less likely but most of all collision much less deadly. It's nice to know your car has power in case you need it, but it doesn't mean you have to use it to the max on a regular basis. Going slower in an ICE vehicle may have limited efficiency benefit since the engine wastes gas even just to idle, but with an electric motor that is much less of an issue I believe, so there is no penalty for slowing down except the time it takes. Leave earlier and bring lots of carrots to chew on as you enjoy music and the scenery.

Agree with slowing down, but not completely with "ICE vehicle may have limited efficiency benefit".

Mythbusters found slowing from 70 to 45mph and using hypermiling techniques, e.g., keeping windows closed, A/C off, coasting whenever possible, drafting, etc, increased their range by 70% in an ICE auto. The myth still ended up being busted since they needed to double their mileage to dispel the myth.

Does anyone remember the Mobile Economy Run? It was a big deal for decades. Professional drivers driving perfectly tuned cars would achieve astonishing MPG by going slow, not accelerating and always "lugging" the motor. One trick was to coast, then accelerate for a minute in high gear, then coast aging. Low air flow produced the leanest mixture with carburetors and the best mileage. I used to do this when trying to reach a gas station while on empty. Mostly it worked.

The cars were strippers, no options and so the lightest weight. In olden days even heaters and the AM radio were optional. A stripper six cylinder car was not that heavy, even with Iron Age engineering.

They had lots of air in the tires and they were prepared like a stock car, except for mileage rather than speed. It would not be surprising if some of them had mileage camshafts, higher compression, blueprinted clearances, etc. Full cheat in other words.

Speaking of hypermiling, here's an interesting Guinness record:

John and Helen Taylor, Australian hypermilers, drove 1,626.1 miles in a 2012 Volkswagen Passat TDI SE from Houston to Sterling, Va., breaking the Guinness record for the longest drive in a passenger car on a SINGLE TANK of diesel fuel.

I probably wouldn't have appreciated being behind them...

I definitely disagree with "ICE vehicle may have limited efficiency benefit"... The same forces of drag apply to an ICE vehicle as an EV. You just don't notice the impact as much because of the ease of refueling.

Whatever the fuel consumption of an idling ICE, it can only be higher when not idling and actually keeping the vehicle in motion, even at low sleep, especially if RPM rises. No? By this logic, the constant speed at which your tank of gas will take you the furthest is probably not very close to 0, the wind resistance may be low, but practically idling the engine is the main power use while your speed is so low. As you speed up, idling loss diminishes as a percentage of total consumption but air resistance increases. There will be a low at the intersection of these two curves. Also, rolling resistance is very significant in cars because they are so heavy, yet they barely vary with speed, so that will rise your entire power graph. So there should be a speed, significantly greater than 0 but less than 100 km/h, where a car is most efficient at constant speed. My physics textbook when I was in high school seemed to suggest that was 70 km/h, but I think it is quite a bit lower although it would vary by model.

Somehow I think an EV is more like a bicycle, where wind resistance is the main thing that changes with speed, so there is no penalty other than time for slowing down. By this same logic, slowing down with an ICE you will eventually find yourself mostly idling and your mileage will be getting worse.

So if an EV is a normal bicycle, an ICE is a bicycle where, whenever you are not pushing the pedals forward to increase or maintain speed, you must pedal backwards and idle. Pretty dumb machine, isn't it?

@GeekEV and others who disagreed with me

Actually I think you are right if you disagreed with me when I said that the curve for an EV and an ICE are significantly different. I think the idling of an ICE can better be visualized as a constant expense for constant RPM, which would mean that the curve would be be similar to that of an EV, except much higher, in other words similar equation, but with + C at the end. Clearly an ICE has a higher loss than an electric motor when idling at a constant and similar RPM, which is why an EV would be the perfect car for hypermiling. Higher RPM would cause more idling loss in both motors, but the rise should be more rapid in an ICE because it also needs to burn fuel at each cycle versus an electric motor that only has to deal with higher friction and air resistance. That's why a standard or manual ICE car is more efficient than automatic, if you keep the RPM rather low.

Anyways, thank goodness for EVs.


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