Optimum speed on the highway

[Cheapergear]

Hi. I just bought a used 2017 BEV. I'm wondering if any of the experienced drivers know the best speed to travel on the highway for range and speed. It's great in city/suburban driving, but the range seems to suffer on the highway. One other question is the steering. It seems very tight and heavy. Is that normal for BMW or just for i3?

 

[derekgates]

I find that above 73mph tends to make my mi/kWh display drop considerably, not to mention the difficulty in steering corrections needed. You can enable the display using the BC button on the top left stalk; it will show the current consumption (or average if you like, using same button).

The drag of the car is pretty extreme; it is not the slickest design. The motor isn't as efficient as Tesla either.

The i3 is also quite aerodynamic with a low drag coefficient of .29.

https://www.plugincars.com/bmw-i3-may-be-best-city-car-market-127857.html

When a Model 3 is quite a bit slicker:

hopefully 0.21

https://twitter.com/elonmusk/status/716751152906440705

So, with the steering difficulty, drag and the display... I try to stay around 70. :lol:

 

[Fisher99]

Interesting comment about "steering difficulty". I recently did 700+ miles in my 2014 i3/REX (with 20" wheels) with the cruise set at 75 for much of the trip, and one short two lane pass hitting 95, and absolutely loved the way the car handled and steered. Which actually amazed me given how quick the steering is at low speeds, making crowded parking lots a breeze. I've driven BMW's of various models continuously since 1971 and have loved the way that every one of them, including this i3, handles at speed.

But to the original question, I don't think I've had enough experience to have identified an optimum speed for maximum battery range. Since I brought the car home (the 700+ mile trip) my driving has consisted of town driving and 10 mile stints where the speed limit is 55.

 

[jadnashuanh]

Drag goes up at an exponential rate...IOW, a small increase in speed means the drag goes up significantly. So, slower is better as is being very consistent as slight speeding up, slowing down means in addition to the drag, the acceleration comes into play as well. FWIW, if you have the ACC, it is better at managing consistency and acceleration better than most people can do, at least long-term.

 

[robthebold]

Drag goes up at an exponential rate...IOW, a small increase in speed means the drag goes up significantly. So, slower is better as is being very consistent as slight speeding up, slowing down means in addition to the drag, the acceleration comes into play as well. FWIW, if you have the ACC, it is better at managing consistency and acceleration better than most people can do, at least long-term.

Right, and that exponent is 2. A ~42% increase in speed (e.g. from 55 to 78, mph or km/h it doesn't matter) increases your aerodynamic drag by 100%. (Your other losses like rolling resistance in the tires and bearings only increase ~proportionally with the speed.) So the decrease in efficiency "increases monotonically w.r.t. speed" as a math instructor would say. There's no point at which this curve levels off. So the optimum speed is . . . as slow as you can go. In the real world you would have to add "and still get there in the time available without getting run over by angry road-ragers".