Why does fuel economy get better on the highway, but EV range gets worse?
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Presumably a gas/diesel car and an EV are up against the same physics factors on a highway – wind resistance, car going faster and needing more power to sustain that speed, etc. So why does the former get better range than local driving, but the latter considerably worse? Logically both should get better or both should get worse with the same conditions.
It’s the advantage of a mechanical transmission. You can drive a higher gear at lower RPMs.
Electric motors are approximately as efficient across their operating range, regardless of engine speed. Electric motors can provide maximum torque from zero RPM. But as you go faster, mechanical friction, rolling resistance and aerodynamic drag all increase, taking a bigger bite out an EV’s stored reserves.
Internal combustion engines (ICEs), on the other hand, must be designed and tuned to run efficiently at a specific engine speed. They are markedly LESS efficient overall the further you get away from this optimized RPM. ICE vehicles make very little torque at idle speeds, and waste a lot of fuel getting up to their operational “sweet spot”, where they make power most efficiently.
In stop-and-go driving, ICE vehicles must constantly transition through these inefficient lower engine speeds. Even though friction and drag losses increase at highway speeds for ICE vehicles just like they do for electric vehicles, they are less evident because the engine remains in that range of optimum designed efficiency.
In short: internal combustion cars have an optimal speed.
Electric cars do not have an optimal speed (or you may say it is zero) – the slower they go, the more efficient they are.
Engines are more efficient at a specific RPM and power output. Cars, between the engine and gearing, are generally tuned to around 50-70mph being the most efficient as that is the typical speed they are driven at that can easily fall into the typical efficient ranges.
That is countered by higher speeds needing more energy to maintain. This is primarily due to wind resistance – doubling speed quadruples the drag – though other friction losses can play a large part as well.
Electric motors don’t have that initial behavior, but do still have to deal with the increasing energy requirements. This means that going faster with an electric system is basically always less efficient, though there is some exception at very low speeds – less than 10mph or so. Electric vehicles also include regenerative breaking, which helps a lot with stop and go movements as well as on downhill roads.
Mostly because EVs have regenerative braking. Both gas and electric cars use more energy to maintain speed on the highway(drag increases with velocity squared). The reason gas cars get worse fuel economy in city driving is that you stop more frequently, and gas cars throw away all the energy they spent accelerating every time they stop, while EVs can recover most of it.
In both cases, the most efficient possibility is driving slowly without braking.
The short answer is combustion engines are super inefficient but they are most efficient at highway speeds so they seem like they make good mileage.
All cars have more drag when going faster because of wing resistance but electric motors don’t get more efficient as they go faster like combustion engines.
EV’s are more efficient overall but as waste increases with speed efficiency drops at high speed, ICE are tuned to be efficient at certain engine speeds.
So this is a non-issue, if not the lack of range of EV’s.
Internal combustion engines have a so called “rev range” in which they operate at. This is the range of engine revolution speeds at which the engine makes usable power, usually expressed in RPM (revolutions per minute), with too low RPM producing too little power to get the vehicle moving (overcome the forces acting against it), and too high RPM reaching speeds where the mechanical components can’t spin any faster and there’s diminishing returns in power, as the engine cannot keep up with the speed to produce more power and there’s a drop off. Usually peak power is achieved shortly before peak RPM. A typical internal combustion engine may have a rev range from ~2k rpm up to 6,5-7k rpm, but those can vary widly depending on the engine type, number of cylinders and layout, fuel type (diesel or gasoline) etc.
What this means is that internal combustion engines have a specific rev range where they’re most efficient and make usable power, and more you rev the engine, the more fuel you’re using. Since the rev range is so limited all ICE vehicles require a gearbox, which changes the gear ratio from the engine to the wheels so that a wide range of wheel speeds can be covered, from slow crawling speeds for slow speed stop and go traffic, up to cruising at highway speeds. Most cars have 5 or 6 speeds, with usually the 4th speed being the 1:1 ratio (or close to that) meaning that the wheels are spinning at the same rpm as the engine. Anything over that is called an “overdrive gear” meaning the wheels are spinning faster than the engine, which ultimately results in a loss of power down to the wheels but allows vehicles to maintain high speeds with low engine rpm, which in turn improves fuel efficiency.
Electric vehicles on the other hand generally don’t have gearboxes, or if they do they’re 1 speed, meaning that they have a single ratio that usually acts as a reduction gearbox to translate the high motor speeds into more usable wheel speeds and get extra power out of the motor. They have a constant torque output, unlike gasoline engines, and if you want to go faster you simply spin the motor faster, but much like gasoline cars this requires more power (just like gas engines need more fuel). With gasoline cars you can tweak your overall engine speed for a given road speed through the gearbox. You could be going 30mph with the first gear near the rev limiter or 30mph with the 6th gear close to stalling. You can be doing 60mph on the highway in top gear with the revs comfortably in the middle. With an electric car, you don’t have that option. If you want to maintain a high speed you have to expend a lot of power constantly for it.
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