Final answer:
A continuously variable transmission (CVT) uses pulleys to change engine speed and provides a continuous range of gear ratios.
Step-by-step explanation:
The type of transmission that uses pulleys to change engine speed is called a continuously variable transmission (CVT).
A CVT uses a system of pulleys and a steel belt or chain to provide a continuous range of gear ratios, allowing the engine to operate at its most efficient speed for a variety of driving conditions. The pulleys in a CVT can change their effective diameter to increase or decrease the gear ratio, without the need for discrete gear steps like in a traditional transmission.
For example, when you accelerate in a car with a CVT, the pulleys can adjust to keep the engine at its peak power while varying the vehicle speed. This helps the engine operate more efficiently, resulting in better fuel economy.