Asking whether maintenance should be scheduled by mileage or by time is the wrong question when it is asked for the whole plan. There is no single answer, because the right criterion depends on the component: oil degrades with use and also with age, brake fluid absorbs moisture even when the vehicle is parked, and a vehicle inspection expires on the calendar no matter how many kilometers have been driven.
The decision, then, is made Service by Service within the plan. A well-built plan combines all three criteria. This guide explains when each one makes sense, what the difference is between a Preventive and an Expiration, and what blind spot makes mileage-based plans fail even when they are well designed.
The first criterion is mileage: the Service is due when the vehicle accumulates a set number of kilometers since the last time that same Service was performed.
The second is time: the Service is due when a set number of months has passed since the last time it was performed, regardless of use.
The third is the combination, usually phrased as “whichever comes first”: the Service is due when either of the two thresholds is met. It is the most widely used criterion in modern maintenance and the one that best protects fleets with mixed usage profiles.
These criteria are defined when building each Service in the plan, within the Maintenance module, and it is worth deciding them explicitly instead of copying the entire manual without reviewing it. How that structure is built is covered in the hierarchy of Services and Tasks.
Mileage applies when wear is proportional to use: friction, heat, operating cycles. The component does not deteriorate while the vehicle is parked. This category includes brake pads and rotors, tires, clutches, belts and, in general, anything that wears through friction.
There is an important caveat: an urban kilometer and a highway kilometer are not equivalent. A delivery vehicle with constant stops and starts punishes brakes and clutch far more per kilometer than a long-haul vehicle. If your operation is mostly urban, the manual’s intervals are too long, and that severity adjustment is defined in the model’s plan, not vehicle by vehicle, as explained in maintenance plans by model.
Time applies when deterioration happens through aging, chemical degradation or environmental exposure, and does not depend on the vehicle being used.
The clearest case is brake fluid, which absorbs moisture from the air and loses its properties while the vehicle sits idle. The same goes for coolant, for batteries, which also suffer when the vehicle goes unused for long periods, and for safety items that have an expiration date, such as the fire extinguisher and first-aid kit supplies.
Time also applies to everything that is regulatory in origin, although those obligations are not preventives but expirations.
It is a distinction worth maintaining from the design of the plan onward, because mixing the two produces confusing calendars.
A Preventive is a technical intervention that you decide on: it comes from the maintenance plan, the company sets the interval based on the manufacturer’s recommendation and its own operating experience, and its goal is to keep something from failing.
An Expiration is an obligation with a date, imposed from outside: documentation, permits, insurance. There is no technical criterion to debate, there is a date to meet, and missing it does not cause a mechanical failure but takes the vehicle straight off the road.
Both coexist in the Calendar, identified as P and V, with a green, yellow or red dot depending on urgency and with the letter T when a ticket is already open. That single view is useful because both compete for the same scarce resource: vehicle availability.
Here is the reason why many well-designed mileage-based plans still fail in practice: the mileage the system sees. In units with integrated GPS it is updated from telematics and triggers the preventives; in units without it, it depends on someone entering the odometer reading.
The plan may say the Service is due at ten thousand kilometers, but without integrated GPS the system only knows the mileage someone entered. If the reading is only updated when the vehicle goes into the workshop, the recorded mileage will always lag behind the actual mileage, and the preventive will show up late by design.
This is worth accepting before defining the plan, not after. If the operation cannot sustain reasonably frequent odometer entries —at the end of the shift, at fueling, in the pre-trip checklist, wherever there is the least friction—, basing the entire plan on mileage means basing it on something you will not have up to date, and the time thresholds become the main safety net.
The practical decision is simple: define who enters the odometer reading, when and how often, before defining the intervals. A plan calibrated on data that nobody enters is not a plan.
Why most Services use “whichever comes first”
The combined criterion handles both extremes of the same fleet with a single definition. The high-use vehicle reaches the mileage threshold long before the time runs out, and the kilometer criterion protects it. The low-use vehicle can go years without reaching the mileage, and that is where the time threshold keeps it from running on oil that has degraded with age.
That is why the combined criterion should be the plan’s starting point, with pure mileage or pure time as deliberate decisions for the components where they clearly apply. It is safer to start from the combination and remove the threshold that is not needed than to start from a single criterion and discover the gap when the failure shows up.
Not every fleet needs the same calibration, and the operating profile is what determines it.
In intensive urban use, such as delivery or last mile, mileage almost always wins over time. What needs to be reviewed there is severity: that kilometer punishes more than the manual assumes. In highway use mileage also governs, but wear per kilometer is lower and the manual’s intervals are usually adequate.
In mixed use, such as distribution or field service, both patterns coexist within the same fleet, and the combined criterion is the only one that covers everyone without individual configurations.
In administrative use and in reserve vehicles, time governs. These are the cases where a pure mileage plan leaves the vehicle without maintenance for years, and where it is worth specifically checking batteries and fluids, which are the ones that tolerate inactivity worst.
When operations force a scheduled date to be skipped, the control can be forced as a manual override. That is legitimate as an exception; if it becomes routine, the problem is not the interval but workshop capacity.
The order that works starts by separating what is not up for debate. First you identify the expirations, which have an imposed date and allow no technical criterion, and you enter them as such. That takes everything regulatory out of the discussion and leaves the preventive plan clean.
Then you classify components by the nature of their wear: those that wear with use, those that degrade with age and those that respond to both. That technical conversation, which is short, settles most of the criterion decisions.
The third step is almost always skipped, and it is the one that decides whether the plan will work: checking how often the odometer is actually entered in your operation. If it is infrequent, you reinforce the time thresholds before going live, not after the first failure.
Once the plan is running, calibration comes on its own. After six or twelve months the history shows how each Service was reached, by kilometers or by date, and which correctives appeared before the preventive that was supposed to prevent them. How to read that relationship between the planned and the unplanned is covered in corrective maintenance and its cost.
