What is the most efficient speed to drive an EV?

Range anxiety is a real thing for electric car owners.

With comparatively less charging infrastructure across the country than petrol stations, the ability to get from one place to another in an EV currently requires a little more planning than it does in a conventional vehicle with an internal combustion engine (ICE) – particularly when driving long distances.

As it is with any car, there are a number of factors that determine how far you can travel before it needs to be refilled – no matter whether it uses petrol, diesel or electrons – such as how powerful the engine is, the geography of the landscape, environmental conditions, your driving behaviour and the speed you travel at.

It’s the latter two elements that will ultimately dictate how efficient your car is in comparison to another driver, and therefore how far you can travel on a single tank or battery charge.

So, let’s drill down and look at what is the most efficient speed to drive an EV to reduce the pressures of range anxiety.

For starters, it is important to understand how an electric vehicle works compared to a car with an internal combustion engine because they operate quite differently, especially in different geographical and environmental conditions.

Basically, an EV has either one or two electric motors that are powered from a lithium-ion battery pack. Each electric motor can produce maximum torque instantaneously and spins at speeds up to 25,000rpm.

This means they have phenomenal and seamless acceleration from standstill and instant response at any speed because most do not require a conventional gearbox like a normal car.

Electric motors also have the unique ability to act as a generator when the car is slowing down (using the kinetic energy of deceleration to produce electricity) that can supply power back to the battery pack. This is commonly known as regenerative braking.

As for the battery itself, it operates differently at different temperatures, which means the driving range will be affected by environmental conditions. Australia’s relatively mild climate is ideal for battery-powered vehicles, as most lithium-ion battery packs are at their most efficient in temperatures between 15-30 degrees Celsius.

The battery will use more energy – while also holding less electricity and be slower to recharge – in both extremely cold and hot conditions.

Taking any of the environmental variables out of the equation, electric vehicles are most efficient in urban conditions.

For starters, the engine doesn’t consume any power when it is not running and can also recoup energy when decelerating. So, an EV is much more effective in stop-start traffic.

However, the laws of physics don’t change between an EV and an ICE-powered vehicle in terms of moving mass because, in both cases, accelerating away from a standstill uses the most energy.

While it is tempting to thrust your head back into the seat and win each green light grand prix in an EV, it is more efficient to use the electric motor’s significant low-end torque to gently accelerate up to speed.

Once on the move on urban roads, maintaining a steady speed is far more efficient than ‘kangaroo-hopping’ on and off the throttle in short, sharp bursts.

However, take advantage of an EV’s regenerative braking abilities to increase your driving range by rolling off the accelerator when travelling downhill and leave enough room to the traffic in front to fully utilise it when coming to a complete stop.

The same rule is even more important out on the open road at highway speeds because this is where electric vehicles consume the most energy.

Unlike a conventional car with a gearbox, which is usually driven on the highway in its tallest ratio and therefore allows the engine to spin at low speeds with small throttle inputs, an electric motor spins faster as speeds increase. And, consequently, uses more battery power.

It is critical, then, to modulate the throttle according to the geography of the landscape in an EV at highway speeds to use less energy and capture as much regenerative braking power as possible.

Obviously, the safest speed to travel is at the posted speed limit. But a few kays less could make a significant difference in making it to the next charging station without inducing a severe bout of range anxiety.

In any case, aerodynamics plays an important role in the efficiency of a car – any car – as speeds increase.

Wind resistance – or drag – increases with the square of the speed of travel, which means that a vehicle travelling at 110km/h creates four times as much force (against the air) as a vehicle travelling at 55km/h.

Wind resistance, generally, starts to have a major effect on the economy of a vehicle at speeds above 70km/h. So, anything you can do to reduce the drag of a vehicle will have a significant benefit when travelling at highway speeds.

Simple things, such as driving with the windows closed and removing roof racks when not in use, will improve the driving range of an EV out on the highway.

Another factor that affects performance – and therefore efficiency – is friction, and there are some easy maintenance measures you can apply to ensure your EV maximises its driving range, no matter the speed.

The first – and most basic – is regularly checking the tyre pressures are optimised. Tyres with lower pressures will create more rolling resistance on the road, which means the engine uses more energy to get up to speed.

It’s a good idea to monitor your tyre pressures every fortnight and keep them always inflated to the manufacturer’s recommended values. Check the placard located on the inside of the door jamb (or the back of the charging flap in some cases), or look in the owner’s manual, for more information.

Other than that, while EVs require less servicing because of the fewer moving parts, it is important to follow the prescribed maintenance schedule for items such as wheel bearings and brakes to ensure the vehicle operates as efficiently as possible.

At the end of it all, there is no ‘one rule’ that determines the optimum speed to drive an EV, as efficiency is ultimately determined by a variety of factors – some of which are totally out of the control of the driver.

Instead, having a better understanding of how an EV works and optimising your driving behaviour to the conditions around you will ensure you get the most from an electric car, both literally in terms of driving range and metaphorically in terms of the driving experience.

Going the distance in an EV – quick tips:
• Gently accelerate away from the lights, happy in the knowledge that you’re probably the quickest
• Maintain a steady speed once you’re up and running, avoiding regular on/off stabs of the throttle
• Roll off the accelerator when going downhill to maximise regenerative braking
• Keep a good distance from cars in front for maximum regen value too
• Wipe off a few kays on the open road, taking it easy on your use of the accelerator
• Reduce aerodynamic drag wherever possible, such as removing roof racks when not in use
• Check your car’s tyre pressures on a regular basis
• Keep your EV up to date in terms of scheduled servicing

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