Service by date or by kilometre: which comes first?

Open almost any service schedule and the intervals come in pairs: a distance and a time, followed by “whichever comes first”. It reads like small print, but it decides when your car is actually due.

Why there are two numbers

Some parts wear with use. Engine oil, filters, brake pads and tyre tread wear out as the kilometres add up, so the distance matters most.

Other parts age with time, however little you drive. Oil breaks down in the engine even on short, gentle trips. Brake fluid slowly takes in moisture from the air, which is why it's usually changed on a time interval alone. Coolant loses its protection. Batteries and rubber parts age whether the car moves or not.

So a schedule gives both, and the service is due at whichever you reach first.

How it plays out

Take an interval written as “every so many kilometres or every year, whichever comes first”:

• If you drive a lot, you'll reach the distance well before the year is up. Your service comes round by kilometre, and a date on the calendar would be too late.
• If you drive a little, the year arrives long before the distance. The car still needs its service, even with few kilometres on it.
• If you're in between, it depends on the month. A long road trip can bring a service forward by weeks.

That's why a fixed date in a diary only works for some drivers, and a sticker with a kilometre figure only works for others.

The parts that age while the car stands

A few things deserve attention on a calendar, not just a mileage:

• Brake fluid, for the moisture it gathers.
• Coolant, which loses its corrosion protection over the years.
• The battery, which weakens with age and with short trips that never fully recharge it.
• Tyres, which harden with age even with tread left. The sidewall carries a four-digit code: the week and year they were made.

Keeping track

The hard part isn't knowing that both numbers matter. It's knowing which one you'll reach first, for every item, given how you actually drive. That means remembering the date of each service, the odometer reading at the time, and how many kilometres you cover in a typical month.

Intervals vary by make, model and engine, and by how hard the car works: towing, heat, dust and short trips all count. The schedule in your owner's manual is the one to follow.