Your PM Schedule Is Probably Wrong for How Your Trucks Actually Run

Here’s a question worth asking about your own fleet: where did your maintenance intervals come from?

For most operations the honest answer is “the manual,” or “what the last guy did,” or “every 15,000 miles because that’s a round number.” All three assume something that is usually false — that a mile is a mile, and that your trucks are doing roughly what the engineers who wrote the interval imagined.

They’re usually not. And the gap between the interval you’re running and the interval your trucks need is where unplanned downtime comes from.

A mile is not a unit of wear

Two trucks, same model, same year, both showing 100,000 miles.

Truck A runs freight on SH-130 and I-10. Long pulls at steady speed, loaded, engine at optimum temperature for hours at a stretch, minimal idling, few cold starts.

Truck B does last-mile delivery in Austin. Forty stops a day, constant restarts, hours of idling in traffic and at loading docks, and an engine that spends much of its life below the temperature it wants.

Those two trucks are not in comparable condition, and no mileage-based schedule can tell them apart. Truck B has:

Far more engine hours per mile
Far more cold starts and heat cycles
Vastly more idle time — which contaminates oil, loads the DPF, and wears the engine while the odometer sits still
Exhaust temperatures that rarely support proper regeneration — the mechanism behind DPF clogging on delivery routes
Much heavier brake wear from stop-and-go

Service both at 15,000 miles and you are over-maintaining one and under-maintaining the other. The over-maintained truck wastes money quietly. The under-maintained one fails on a route.

Use hours, or hours and miles, whichever comes first

The fix is not complicated. Set intervals on engine hours, or on hours-or-miles-whichever-comes-first. Every modern ECM tracks hours and idle percentage, and pulling that data is a fast job.

Pull it for your fleet before you set anything. Two numbers matter:

Idle percentage. Most operators guess low. Delivery, vocational, and service fleets frequently discover that a large share of engine hours were spent stationary. That time is doing real damage and contributing nothing to the odometer.

Hours-to-miles ratio. This single number tells you which category each truck belongs to. Compute it per truck, not per fleet — a mixed fleet often has trucks at both extremes on the same maintenance schedule.

Once you can see the spread, the trucks sort themselves into groups, and the groups want different intervals.

What changes, by duty cycle

Stop-and-go and delivery

Shorter oil intervals — idle time contaminates oil on a clock, not an odometer. Oil service is the cheapest insurance in the fleet.
Much shorter brake intervals. Stop count, not miles, drives brake wear.
Planned DPF service rather than waiting for a derate. These trucks structurally struggle to regenerate, and pretending otherwise means the truck picks the timing.
Battery and charging attention — many starts, lots of accessory load.

Vocational and construction

Everything above, plus dust and load cycling. Air filtration, cooling stack cleaning, suspension, and frame inspection move up sharply. Covered in detail in what the 130 buildout did to construction fleets.

Highway and corridor

The easiest duty cycle on aftertreatment and brakes, and the hardest on tires, cooling, and anything already marginal at sustained load — see what the SH-130 corridor does to a diesel. Mileage-based intervals actually work reasonably well here, which is precisely why they became the default and why they mislead everyone else.

Mixed fleets

Most Central Texas fleets are mixed, and the common mistake is picking one interval for everything. Group the trucks by what they do, not by what they are. Two box trucks doing very different work belong on different schedules.

Seasonality is part of the schedule

Texas has two maintenance seasons and most fleets ignore both.

Before summer: cooling systems, A/C, and anything heat-sensitive. Heat is what kills batteries here, and a battery that dies in January was killed the previous August. Do this in spring, when shops have capacity — see Texas summer and cooling systems.

Before winter: batteries load-tested, water separators drained, cold-start systems verified, anti-gel additive on the shelf. It’s a short cheap list that turns the occasional hard freeze from an all-hands emergency into a normal week — the freeze problem.

Neither is a big program. Both are usually skipped because nobody owns the calendar.

The objection, answered

“Shorter intervals cost more.”

They cost more in scheduled maintenance, which is the cheapest money a fleet spends. What they save is unplanned downtime, which is the most expensive — because it includes the tow, the emergency rate, the missed route, the driver standing around, and sometimes a customer.

Correcting intervals also isn’t uniformly shorter. Highway trucks are frequently being serviced more often than they need. The point isn’t more maintenance, it’s maintenance aimed at where the wear actually is — and that usually means moving money from trucks that don’t need it to trucks that do.

What this looks like in practice

Pull ECM data on every unit. Hours, miles, idle percentage, fault history.
Group by duty cycle, not by model year or vehicle type.
Set intervals per group, on hours or hours-or-miles.
Add the two seasonal cycles.
Track it somewhere that isn’t a person’s memory. This is where most programs fail — not on the intervals, but on nobody owning them.
Revisit annually. Duty cycles change when routes and contracts change, and the schedule should change with them.

Step 5 is the one that quietly decides whether any of this works. Interval math is easy; consistently executing it while running a business is not.

Or hand it to us

This is exactly what a fleet maintenance program is for. We pull the data, set intervals against how your trucks actually run, schedule the work, and track it — including the seasonal cycles and DOT inspections before they come due. With concierge pickup and delivery, your drivers never make a shop run for any of it.

If you’d rather keep your own fleet manager and just want the backup, that’s fleet maintenance support. And if you’re weighing what running this in-house actually costs, we wrote that comparison out in fleet management cost vs. a fleet manager’s salary.

Call fleet sales at 512-273-7351.