When to Schedule PM by Duty Cycle, Not Just Mileage

"Every 15,000 miles" is an easy number to put on a calendar, and for a truck running long, steady highway miles it's a reasonable approximation. But mileage alone doesn't capture what actually wears a truck out — and for a lot of fleets, going by mileage means catching problems too late or servicing units that didn't need it yet.

Why mileage alone is misleading

A truck running short urban routes with constant stop-and-go traffic puts far more stress on brakes, the clutch, and the drivetrain per mile than one running steady interstate hauls. A truck idling for long stretches — refrigerated delivery, waiting at docks — accumulates engine hours without accumulating comparable mileage. Two trucks with identical odometers can be in very different mechanical condition.

What "duty cycle" actually means

Duty cycle is the real-world pattern of how a unit is used: route type (highway vs. urban), load weight, idle time, stop frequency, and climate exposure. A maintenance schedule built around duty cycle adjusts intervals based on these factors instead of treating every truck the same.

• Highway-heavy routes: longer intervals are often appropriate, but tire and brake wear should still be checked on schedule
• Urban / stop-and-go routes: shorter brake and drivetrain inspection intervals
• High-idle duty cycles (reefer, PTO-heavy work): engine hour tracking alongside mileage
• Severe conditions (gravel, extreme cold, heavy loads): tighter intervals across the board

Where the federal minimum still applies

Whatever the duty cycle, the federal periodic inspection requirement under 49 CFR § 396.17 is a floor, not a target — at least once every 12 months, full stop. Most fleets running a real duty-cycle-based PM program are inspecting far more often than that as a byproduct of the schedule itself.

Building the schedule

In practice, this means starting with the manufacturer's baseline intervals, then adjusting up or down per unit based on how it's actually run — not applying one number fleet-wide. It's more setup work up front, and it's the difference between a PM program that catches problems early and one that's really just an oil-change schedule with a compliance form attached.