How to build PM intervals for mixed fleets (hours, miles, cycles)
When one fleet has on-road trucks, off-road equipment, and trailers, the 'common PM interval' trap costs you. Here's how to set intervals per asset class without doubling your admin load.
Most mid-sized construction fleets are mixed: a couple of on-road dump trucks racking up miles, half a dozen excavators and loaders racking up engine hours, a stack of trailers and light towers racking up calendar time. The temptation is to give the whole fleet one PM rhythm — “every 90 days” — because it's easy to explain. It is also the most expensive shortcut in fleet ops. Here is what to do instead.
The mixed-fleet PM problem
Imagine a 25-asset fleet: 4 on-road trucks, 12 pieces of off-road equipment, 6 trailers, 3 light towers / gensets. Three units of wear are in play at the same time:
- Miles on the on-road trucks — drivetrain, tires, brakes, suspension wear all scale with road use.
- Engine hours on the off-road equipment — engine, hydraulics, attachment cycles all scale with time the engine is running, not distance.
- Cycles or calendar days on trailers and standby gensets — there is no meter, but UV exposure, tire dry rot, seal hardening, battery sulphation all tick along on the calendar regardless of use.
Set one interval across all three groups and you guarantee waste somewhere and risk somewhere else.
Why “every 90 days” fails three different ways
Take the 90-day rule and apply it to each asset class:
- The dump truck running 8,000 miles a quarter blows past its real 5,000-mile service interval halfway through. The brakes glaze, the differential oil cooks, the warranty claim gets denied because you didn't hit the OEM cadence.
- The excavator that ran 720 hours in a hot quarter is 220 hours overdue on the 500-hour PM. The same 90-day-old PM is wasteful on the excavator that only logged 110 hours during a rainy quarter — you change oil that is barely used.
- The trailer that hasn't moved all winter still has tires aging in the yard and brake actuators seizing. The 90-day inspection might catch it; it might not, and nothing on the trailer screams unless you go looking.
The common interval is not even a compromise. It's wrong everywhere at once.
The 3-axis approach
Set the trigger to match the wear mechanism, per asset class. The three axes most mixed fleets need:
| Asset class | Primary PM trigger | Secondary trigger | Typical cadence |
|---|---|---|---|
| On-road dump trucks / pickups | Miles | Calendar (12-month inspection) | 5,000 / 10,000 / 25,000 mi |
| Excavators, dozers, loaders | Engine hours | Annual safety inspection | 250 / 500 / 1,000 h |
| Skid-steers, mini excavators | Engine hours | Annual | 100 / 250 / 500 h |
| Trailers | Calendar | Annual DOT inspection | 90-day visual / annual DOT |
| Standby gensets, light towers | Hours run | Calendar (monthly start) | 250 h or 12 months, whichever first |
| Mixed-use trucks (vocational) | Miles AND hours (whichever hits first) | Annual | Per OEM dual schedule |
Setting per-class baselines
Manufacturer recommendations are the starting line, not the answer. The OEM spec is written for clean, light-duty conditions on a brand-new machine. Your fleet runs in sand, salt, dust, or freeze-thaw — adjust:
- Severe-service multiplier.Most OEMs publish a “severe service” column that cuts the interval by 25-50%. If you run on a quarry or a coastal site, you are in the severe column whether you want to be or not.
- Oil sample analysis (UOA). Two consecutive samples on the long edge of the interval gives you license to extend; two on the short edge tells you to shorten or change brand. Cheap insurance.
- Reality check against your failure log. Pull the last 12 months of repairs by asset class. If you see hydraulic-pump failures clustering around 1,200 hours, your 1,000-hour PM is too late; pull it forward to 800.
- Crew capacity.If your shop can only turn over 6 PMs per week, an aggressive interval that schedules 10 won't happen anyway. Tune to what the shop can actually deliver.
Running them together without a spreadsheet of doom
The reason most fleets give up and apply a common interval is that running per-class intervals manually means a spreadsheet that grows a tab per asset class, a calendar view that nobody updates, and a foreman who memorizes which truck is due when. Three things make it tractable:
- Centralize the meter feed. One place where the driver or operator logs hours / miles at shift end. The log-hours screen handles both: hour meter on the equipment side, odometer on the truck side, same single screen.
- Per-asset PM intervals, not per-fleet. Each asset record stores its own thresholds. Truck #7 flags at 5,000 miles since last service; excavator #11 flags at 500 hours since last service; trailer #03 flags at 90 days. The system does the comparison; the foreman looks at one flag list.
- One flag inbox. When the meter crosses a threshold, an AUTO_PM flag opens with the right severity. The shop manager works the inbox in order; they never have to remember whose interval is whose.
How DirtFleet handles it
Every asset in DirtFleet has its own PM configuration: primary trigger (hours, miles, or days), secondary trigger (optional, whichever-hits-first), threshold value, and severity. The same hours-log entry that feeds cost-per-hour also drives the PM threshold check. The same calendar clock that drives expiry of certs and inspections drives the trailer's 90-day flag. One inbox, one workflow, per-asset rules — and a flat organization priceso a 25-asset mixed fleet doesn't pay a per-truck and a per-excavator and a per-trailer line item.
The hours-vs-mileage choice itself — when each one is the right trigger — gets its own write-up: PM by engine hours vs mileage. And the calendar-vs-hours angle covered in hour-based PM beats calendar PM is the third leg of the same stool.
The summary
A mixed fleet needs mixed intervals. Pick the trigger that matches the wear mechanism for each asset class, let manufacturer specs and your failure log set the threshold, and put one inbox in front of the shop manager so it doesn't become a clerical job. The common interval is the most expensive shortcut you can take — skip it.