Final answer:
The main indicator for knowing when to service a forklift, according to the suggested service schedule, is the hour meter reading, which tracks the usage hours of the machine.
Step-by-step explanation:
The primary indicator that determines when a forklift is ready for service, when combined with the forklift's suggested service schedule, is D) Hour meter reading. The hour meter on a forklift tracks the cumulative hours the machine has been used, and service schedules are often based on these readings rather than other indicators such as tire pressure or fuel consumption. Regular maintenance based on the hour meter ensures the forklift remains in good working condition and can help prevent breakdowns or accidents due to equipment failure.