EV Efficiency & Range at 60–65 mph behind Large Vehicles vs 60–65 mph Unshielded vs 70 mph Unshielded

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This is part of an article originally published on EV Obsession.


On 1 October this year, I published an article in CleanTechnica in which I shared my experience of driving an EV. I explained the facts and theory behind getting the best range out of an electric vehicle. The title of the article was “Electric Car Range Tips, & How An Electric Car Works.” I originally intended to call it “EV Experience,” but the longer title was chosen for the sake of search engine optimisation.

There was much interesting discussion below that article. One useful part of the discussion was someone asking me for precise figures in the difference, in range, between driving slower, driving faster, and driving behind a large vehicle to reduce air resistance. I had to confess that I couldn’t give him precise figures, as I had based my article just on my experience of driving an EV for a couple of years and noticing a definite difference in range in those three scenarios. I decided that, rather than leaving it open, I would perform some road trials to see exactly what the difference was.

Trial Protocol

My electric car has an indicator for battery charge level, which mimics a fuel gauge. It shows bars that slowly disappear, one by one, as the level goes down. When the car gets to the last three, they start to flash on and off to tell me that I am nearly running out of energy, as if I did not already know. That would not be much use for giving precise figures for battery level and ranges.

Getting the Figures

I have noticed that when I charge up using Ecotricity fast-chargers on the motorway (freeway), the display on the charger includes a precise figure for state of charge, expressed as a percentage. What I decided to do was to drive from my home to a fast charger on the M1 motorway (not far from where I live), connect my car to the charger, and charge it to 80%. This is the point at which it automatically switches off in any case, so it was easy to get a precise starting point of 80%.

I took photographs of the charger display, and I then drove south on the motorway until I reached the next charging point, about 23 miles away. There, I connected the car to the charger again and photographed the display on the charger, showing the precise percentage of charge remaining at the beginning of that charge session. By comparing the two, I was able to provide a precise figure for the percentage of charge used in travelling the 23 miles.

Getting the Comparisons

I performed this operation on three separate occasions. The first time, I drove behind various large vehicles, including a bus, at speeds of between 60 and 65 mph. On the second occasion, I drove at speeds between 60 and 65 mph, in roughly the pattern of variation that I remembered, but avoiding travelling behind any other vehicle. On the third occasion, I drove at 70 mph, the maximum speed allowed on the motorway — again, without travelling behind any other vehicle. By doing this, I arrived at the precise figures that I had been asked for.

I also made images of the emails from Ecotricity, detailing a record of each particular charge event. Those emails provide independent corroboration of my statement that I did charge at those particular places on those particular days and times.

Before anyone starts to think how dedicated I must be to do all of this driving just to get figures for my readers, I did this on three separate days, when I had other reasons for driving along that stretch of the M1. The only extra trouble was in making the extra charging stop to charge up to the 80% first, and in photographing the displays. I was interested to find out for my own sake, too, just to make sure that I was not, unnecessarily, restricting my view of an otherwise panoramic motorway landscape to the back end of a bus. I also wanted to know just how much difference it would make to be driving at the maximum motorway speed, rather than slower, range-saving speeds.


The Ideal World

I would be the first to admit that this is not a precise scientific study. To get something more precise, I would have needed to drive on a circular test track, all on the same day, where no wind and temperature differences could affect the figures, and where speeds would be very precise and constant and any shielding vehicle constantly present or not.

The Real World

Obviously, driving on the motorway on three different days, there might have been some small differences in wind direction and strength and in the air temperature, but not significantly so. Also, I would have been very lucky to have found a large vehicle to drive behind from the start, and to remain behind it constantly throughout. There was a short period with no vehicle in front, at the start, and as the first vehicle I found was driving rather slower than I required, I switched to another, after a few miles.

When trying to drive without any vehicles in front, but at the same average speed as before, and having due consideration for other drivers on the road, I was not always able to do so. As these defects reduce, rather than increase any differences, I am not too concerned about them. Finally, one last problem was that on the last occasion, the charger comms failed. As I did not get the usual emailed bill, you will have to take my word for it.

The Data

Copies of images of figures used

The Peugeot Ion has a 16 kWh battery, which is where the “16” comes from in the table below. The “Distance in Miles,” of 23 miles, is the 22.6 miles taken from the sat-nav and the car’s own trip counter, and then rounded up.

Battery Capacity kWh
Start %
End %
Distance in Miles
Driving Style
Shielded 60-65 mph
Unshielded 60-65 mph
Unshielded at 70 mph

The Results

Below, the “% Used” is simply 80% minus the “End %.” The “kWh Used” is calculated as the “% Used” of 16 kWh.  The “Miles per kWh” is the 23  miles, divided by the “kWh Used.” The “Calculated Range in Miles” is the range from 100% charge, assuming 16 kWh is available from the battery, and so is 16 multiplied by the “Miles per kWh”.

% Used
kWh Used
Miles per kWh
Calculated Range in Miles
Driving Style
Shielded 60-65 mph
Unshielded 60-65 mph
Unshielded at 70mph

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Andy Miles

As a child, I had the unrealistic expectation that I would learn about, and understand, absolutely everything during the course of growing up. Now, at the other end of life, I am fully aware of how much I have not learnt and do not understand, and yet, I remain interested in everything. My education, starting with an arts degree and going on to postgraduate studies in everything from computer science to hypnotism reflected my broad interests. For 20 years, I worked in local government. I am now retired, living in North Leicestershire in the UK, with plenty of time for doing whatever I like. I have always had a keen interest in everything alternative, which includes renewable energy and energy efficiency and, of course, electric vehicles. So, naturally, I have taken ownership of an EV, now that they are affordable and practical forms of transport. Writing is also one of my great pleasures, so writing about EVs and environmental issues is a natural evolution for me. You can find my work on EV Obsession, and CleanTechnica, and you can also follow me on twitter.

Andy Miles has 49 posts and counting. See all posts by Andy Miles