Which transmission type is known for potentially better fuel economy by keeping the engine at optimal RPM?

The transmission known for potentially improving fuel economy by keeping the engine near its optimal RPM is a continuously variable transmission (CVT).

A CVT can alter its ratio continuously rather than shifting through a small number of fixed gears. That lets its control system choose a ratio that holds the engine near an efficient operating point while the vehicle’s road speed changes. In ordinary driving, the engine may therefore avoid repeatedly climbing and dropping through conventional gear ratios.

The benefit is potential, not a guarantee. A CVT’s advantage depends on the engine, software calibration, vehicle design, terrain, and driver behavior. Mechanical losses, engine noise, and the compromises needed for acceleration can reduce or even eliminate the economy advantage in particular vehicles. Traditional automatics and dual-clutch transmissions can also be highly efficient, especially with many closely spaced gears.

The phrase “optimal RPM” is a useful simplification: efficiency depends on engine load as well as speed. A well-controlled CVT seeks a favorable combination of both, not merely one magic rotational speed.