What is the significance of the tachometer's display?

Answer: C — To show engine speed

The tachometer's display is significant because it accurately indicates the engine speed, which is measured in revolutions per minute (RPM). This information is crucial for drivers to understand how hard the engine is working, allowing them to operate the vehicle efficiently and prevent engine over-revving. Maintaining the engine within the optimal RPM range helps enhance performance, fuel efficiency, and longevity. Having the right RPM is vital for shifting gears, as it ensures smooth transitions and prevents strain on the engine and drivetrain. Monitoring engine speed also aids in understanding the engine's performance during various driving conditions, making it an essential instrument in vehicles, particularly heavy-duty ones like tractor trailers and HEMTT.