The 90 Percent Plateau: Why an Electric Motor's Efficiency Curve is Broad and Flat Compared to an Internal Combustion Engine

Ask a traditional driving instructor or a combustion-engine mechanic how to maximize fuel economy, and they will give you the same answer: get the car into its highest gear and cruise at a steady 55 miles per hour. That claim is entirely accurate for an internal combustion engine, which requires a highly specific combination of rotational speed and load to extract the most work from a drop of fuel. But the physics of electric vehicles set that rule directly against the evidence. According to a September 2024 data release from the Department of Energy, a typical electric vehicle converts 87 to 91 percent of its electrical energy into wheel-turning power, compared to just 30 percent for a conventional gasoline vehicle. The difference is not just the higher peak number; it is the shape of the efficiency curve itself.[1][3]

To understand what this means for a driver deciding between a gas-powered SUV and its electric equivalent, you have to look at an efficiency map. An engine's efficiency map is a topographical chart plotting rotational speed (RPM) against torque (rotational force). For a gasoline engine, the map resembles a steep mountain. The "sweet spot" is a tiny peak located at a very specific RPM and throttle position. "Meanwhile, for the combustion engine, the efficiencies do not go higher than 34% with an average efficiency of around 20%," wrote Tatiana Santos Andrade, lead researcher at Chalmers University of Technology, in a 2024 powertrain analysis. If you are stuck in stop-and-go traffic, or accelerating hard up a hill, the engine falls off that peak, and efficiency plummets.[4]

This narrow peak is the entire reason multi-speed transmissions exist. A modern eight- or ten-speed automatic gearbox is essentially a mechanical workaround designed to artificially keep the engine's RPM pinned to that tiny efficiency mountain, regardless of how fast the wheels are actually turning. Every time the transmission shifts, it is fighting to prevent the engine from bogging down into an inefficient zone. For the owner, this means paying for, maintaining, and eventually repairing thousands of moving parts just to compensate for the engine's inherent inflexibility.[3]

An electric motor's efficiency map, by contrast, does not look like a mountain. It looks like a massive, flat plateau. Alternative current (AC) electric motors, particularly those using permanent magnets, maintain an efficiency of 85 to 90 percent across almost their entire operating range. Whether the motor is spinning at 1,000 RPM while you navigate a parking lot, or 10,000 RPM as you merge onto the interstate, it remains highly efficient. The US Environmental Protection Agency's eMotor ALPHA maps confirm that this broad efficiency band persists across varying loads and speeds, effectively decoupling the vehicle's speed from its powertrain efficiency.[2][3]

This flat curve explains why the vast majority of electric vehicles on the market today use a single-speed transmission. Because the motor does not need to be artificially kept in a narrow sweet spot, there is no need to shift gears. The motor simply spins faster as the car accelerates, delivering seamless torque while remaining on that 90 percent plateau. For the buyer, this eliminates the transmission as a point of failure and removes the jerky hesitation of gear changes.

The physics behind this plateau come down to how the two systems generate force. An internal combustion engine relies on thousands of controlled explosions per minute. It loses massive amounts of energy to heat, exhaust gases, and the mechanical friction of pistons scraping against cylinder walls. An electric motor generates rotational force through electromagnetism. A stationary stator creates a rotating magnetic field that drags the rotor along with it. Because there is no physical contact between the stator and the rotor—only a magnetic hand-shake—mechanical friction is virtually eliminated.[1]

The only significant losses in an electric motor come from electrical resistance in the copper windings (which generates some heat) and magnetic core losses. However, these losses scale predictably and remain relatively small compared to the total power being delivered. In a comprehensive analysis of the Worldwide Harmonised Light Vehicles Test Cycle (WLTP), researchers found that an electric machine's efficiency rarely dips below 70 percent even in the least optimal conditions, whereas a combustion engine averages around 20 percent efficiency across the same urban and highway driving mix.[2][4]

For the person plugging their car into a garage wall charger every night, the broad efficiency curve translates directly into predictable range and lower electricity bills. It means that driving aggressively or crawling in traffic does not penalize the powertrain's efficiency the way it does in a gas car. While aerodynamic drag will still reduce an EV's overall range at highway speeds, the motor itself is not working any less efficiently to turn the wheels. The 90 percent plateau ensures that almost every cent spent on electricity goes directly into moving the vehicle forward.[1][4]