Tire selection is on of the most important aspects. Only this changes rolling resistance (plus ideal pressure).
Having more efficient wheels (all four) has more effect than rear wheel arch covers. Cheapest option is to take non-alloy wheels and add almost flat plastic hubs. More expensive is to find some shiny covers:
Short range EVs <30kWh -- Medium range: 30-60kWh -- Long range: >60kWh Charging: Trickle <3kW -- Normal 3-22kW -- Fast 50-100kW -- Supercharging >100kW