Why does fuel economy get better on the highway, but EV range gets worse?
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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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