Most Efficient Freeway Speed...Is There One?

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BeachLeaf

Active member
Joined
Jan 23, 2019
Messages
43
Location
San Luis Obispo
Hi all, on a recent road trip and riddled with range anxiety from not having made the trip before, I wondered if anyone has found what the most efficient highway/freeway speed is?

That is, is there a notable difference from going 55 MPH vs. 65 MPH when considering distance?

Thanks,

Mark
 
The most efficient speed is something like 17MPH. That being said, 55MPH is definitely better than 65MPH, and 40-45MPH works well on secondary roads . Since the indicated speed is 2MPH high, I usually set (on my old 24kwh Leaf) the cruise control to 57MPH, and let it drop to 50 on uphills. The bottom line: if you need to extend range, go as slowly as traffic safely allows. If you have to go fast, try getting 4 car lengths behind a tractor-trailer, matching speeds, and drafting it. That isn't extremely safe, however...
 
In the Okanagan Valley there are few speed limits over 90 kmh. And the Mounties live their speed traps. Sooooo, we spend a lot at around 90 to 95 kmh. Our GOM is typically more than 190 on a full charge and it’s accurate. So yah. I would say 55 mph is quite a bit more efficient.

Jmho.
 
As Leftie said, the slower the better but of course you don't to be the only one going 50 or 55 if everyone else is doing 70mph. I've noticed a much faster drop in charge going over 60 so if you can do 55 that would probably be best if traffic allows, 60 if not. With the Leaf(and probably all EVs) side roads where the speed is slower is better than freeway speeds, even if you have to stop for signs or lights.
EVs really shine in stop and go traffic, getting better than 70 with no traffic, when driving my EV it's the one thing positive to say about slow traffic :)
70 is a real range killer but if I have plenty of charge or a short distance the Leaf handles 70+ just fine, just kills range.
 
The force of the air slowing the car down varies by the square of the velocity, so the speed of the vehicle has a huge impact on efficiency.

For example, 50 mph is about 22.4 m/s and 60 mph is about 26.8 m/s, do the math and, all other things being equal, you'll find that 10 mph change increases air drag force by 43%
 
I have found that the best speed to highway traffic flow is to set the cruise control to 53-55.

That is the national speed limit. If you are on an interstate, and the speed limit is 65-70, then stay at 53, and put on your hazard lights.. You have the lawful right to go any speed you want, so let them be annoyed.

You are not going to lose to keep strangers that are speeding happy.
 
powersurge said:
I have found that the best speed to highway traffic flow is to set the cruise control to 53-55.

That is the national speed limit. If you are on an interstate, and the speed limit is 65-70, then stay at 53, and put on your hazard lights.. You have the lawful right to go any speed you want, so let them be annoyed.

You are not going to lose to keep strangers that are speeding happy.

The Highway Code says:

“Hazard warning lights. These may be used when your vehicle is stationary, to warn that it is temporarily obstructing traffic. Never use them as an excuse for dangerous or illegal parking. You MUST NOT use hazard warning lights while driving or being towed unless you are on a motorway or unrestricted dual carriageway and you need to warn drivers behind you of a hazard or obstruction ahead. Only use them for long enough to ensure that your warning has been observed.”

IMHO people driving 70 in a 70 zone are much safer and lawful (though not more efficient) than a car going 53.
 
IMHO people going 15MPH above the speed limit are endangering others as well. If the posted limit is 70, that's another story. In the Northeast, people usually drive much faster than the posted limit.
 
So what people are saying is that if the speed limit is 65, then everyone needs to do this speed (or more)?

Anyway, I found that a good number of states allow Hazard lights to be used while driving.....

https://drivinglaws.aaa.com/tag/hazard-light-use/

I think that if a person does not feel comfortable driving faster than 55, then no one has the right to critique that, no matter what the speed LIMIT is... I have the right to drive at any lawful speed I want. So, I think 55 is still good on ANY highway...
 
LeftieBiker said:
The most efficient speed is something like 17MPH. That being said, 55MPH is definitely better than 65MPH, and 40-45MPH works well on secondary roads . Since the indicated speed is 2MPH high, I usually set (on my old 24kwh Leaf) the cruise control to 57MPH, and let it drop to 50 on uphills. The bottom line: if you need to extend range, go as slowly as traffic safely allows.
The most efficient speed is assuming no accessory power use, and there isn't a lot of change between the most efficient speed and about 50-60 km/h (30-35 MPH). If you are using heat or AC, the top end of that range can be more efficient because you get where you are going faster and use less energy for climate control. Beyond about 60 km/h the drop in efficiency with increasing speed becomes noticeable, and continues to get worse the faster you go.

LeftieBiker said:
If you have to go fast, try getting 4 car lengths behind a tractor-trailer, matching speeds, and drafting it. That isn't extremely safe, however...
Will adaptive cruise control on the newer models allow you to get you close enough to make a difference with less reliance on the driver's reaction time?
 
powersurge said:
So what people are saying is that if the speed limit is 65, then everyone needs to do this speed (or more)?
Speed differentials are the biggest speed-related traffic hazard, so while people shouldn't drive over the speed limit, it isn't a good idea to go much slower either, unless road conditions are poor.

It really doesn't help that most freeways in North America were designed for speeds of 130 km/h (85 MPH). Enforcing limits much below that will always be problematic because it just feels too slow to most drivers. A lot of money could have been saved over the years by using a slower design speed, which would have allowed narrower lanes, shorter merge and exit lanes, sharper corners and steeper hills. Those features would have also cued drivers to slow down, and enforcing slower, more efficient speed limits would have a lot more support.
 
powersurge said:
So what people are saying is that if the speed limit is 65, then everyone needs to do this speed (or more)? ...

That is the standard interpretation of speed limits by American drivers. I.e., "At LEAST drive the speed limit! What are you, crazy?" :lol:
 
In my gen 1 (2013), agreed that best to not stay above 55 unless I’m a steep downhill section, and let it drift to 50mph on inclines.

Enjoy the right lane.

Keep tires at 40psi or a bit more as you feel that small hike in range from 35psi.


Watch for thick pockets of traffic to maintain momentum. It’s better to glide in neutral then to regen if continuing on the highway. Get used to the quick wrist manuver of flipping the car out of drive. Would stay away from b mode on highway as it’s too easy for regen to kick in.
 
Will adaptive cruise control on the newer models allow you to get you close enough to make a difference with less reliance on the driver's reaction time?

Yes, approximately. The default setting of an alleged 3 car lengths may be too long (it's more like 5 or 6), but you can change it to two or one car lengths (which is actually about 3-4 small car lengths). The way that Pro Pilot ACC reacts, though, is less than ideal. It will wait a bit too long, brake HARD, and then, when the way is clear, it accelerates too slowly. I wouldn't want to rely on it alone if drafting a truck.
 
I have noticed that many of the review sites are quoting Leaf+ performance at 70 MPH. Efficiency only in the mid 3's of miles per kWh. While that may be typical for many consumers, I would like an "economist" of EV drivers site to show the more thrifty range/efficiency for different evs. I am guessing the different cars perform quite differently at different speeds in terms of relative efficiency.
 
DougWantsALeaf said:
I have noticed that many of the review sites are quoting Leaf+ performance at 70 MPH. Efficiency only in the mid 3's of miles per kWh. While that may be typical for many consumers, I would like an "economist" of EV drivers site to show the more thrifty range/efficiency for different evs. I am guessing the different cars perform quite differently at different speeds in terms of relative efficiency.
I'm not sure what you mean but Aero drag is proportional to CdA and to the square of velocity. This means that if you take two cars and set them to the same speed on the same road (to have the same air density) the drag will vary by their relative CdA values. Different speeds will not change CdA(1)/CdA(2)
 
OMG now we are debating drag coefficients!!

The original question was is there a best speed for efficiency.... I say..... Keep it between 50 and 60...
 
BeachLeaf said:
That is, is there a notable difference from going 55 MPH vs. 65 MPH when considering distance?
Mark
~ 20%. Is that 'notable' ?:

Aero drag is 0.5 * Rho * Cd * A * v * v

For the 24 kWh LEAF,
CdA * Rho * 0.5 = 0.725 * 0.6125 = 0.44

Air drag at each speed:
55 mph = 24.6 m/s. At 55 mph there are 24.6 * 24.6 * 0.44 = 266 Newtons of air drag
65 mph = 29 m/s. At 65 mph there are 29 * 29 *0.44 = 370 Newtons of air drag

--> 370 - 266 = 104 Newtons difference. Over one km that is 104*1000 = 104k joules/km = 29 Wh/km. Or 46 Wh/mile.
Or perhaps more useful, 4.6 kWh more energy to the wheels per 100 miles. That would be ~ 4.6/0.87 = 5.3 kWh more battery energy
 
DougWantsALeaf said:
I have noticed that many of the review sites are quoting Leaf+ performance at 70 MPH. Efficiency only in the mid 3's of miles per kWh. While that may be typical for many consumers, I would like an "economist" of EV drivers site to show the more thrifty range/efficiency for different evs. I am guessing the different cars perform quite differently at different speeds in terms of relative efficiency.

Yah. 70 mph is somewhere around 115 kmh. Few and far between in this province. Most places it’s 90 or 100 kmh.
 
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