The dual penalties of highway speed and cold
EV efficiency is measured in miles per kilowatt-hour, and this metric collapses dramatically on highways and in winter. At 55 mph, an EV might achieve 4 miles per kWh; at 70 mph, that drops to 3 miles per kWh; at 80 mph, it approaches 2 miles per kWh. Highway drag increases with the square of velocity, so the efficiency penalty accelerates at higher speeds.
Cold weather compounds the loss. Temperatures below 32°F reduce battery efficiency directly and increase cabin heating load, which draws power from the same battery pack. A car rated for 300 miles in summer conditions might achieve only 200 miles in winter at 70 mph, roughly a 30-40% overall hit when both factors combine.
How real-world conditions differ from the EPA label
EPA testing occurs in controlled lab conditions at moderate speeds and temperatures. Highway driving at 75 mph in February is dramatically more demanding than the combined city/highway test cycle, so achieving rated range requires either moderate speeds (55-60 mph) or mild weather. A car rated at 300 miles has probably never achieved that distance on a representative highway road trip.
This gap between labels and real-world use is the core reason long-distance EV travel requires trip planning and acceptance of longer charging stops. A 300-mile rated vehicle planning a 250-mile highway journey in winter should add a 30-minute charging stop or face arriving at the destination with dangerously low battery, a problem gasoline cars with large fuel tanks sidestep.