The automotive term for a gear ratio that lets the engine turn slower than the driveshaft during cruising is overdrive.
In an overdrive ratio, the transmission output rotates faster than the transmission input. In a conventional rear-wheel-drive layout, this means the driveshaft can turn faster than the engine, reducing engine revolutions for a given road speed. Modern usage also describes any high gear with a ratio below 1:1 as an overdrive gear.
Lower engine speed can reduce fuel consumption, mechanical wear, and cabin noise, especially during steady highway driving. The benefit is not automatic in every situation: if the engine is operating outside its most efficient load range, or if the vehicle is climbing or towing, the transmission may shift back to a lower gear.
Older vehicles often used a separate, bolt-on overdrive unit. As transmissions gained more gears, manufacturers generally integrated overdrive ratios into the main gearbox. The term is therefore not the same as kickdown, which commands a downshift for stronger acceleration, or underdrive, which increases engine speed relative to the output.