Your representative car logbook period

What the law asks

The Income Tax Assessment Act 1997 asks for a logbook in the first income year you use the logbook method for a car (section 28-115), covering a continuous period of at least 12 weeks (section 28-120). The percentage you claim is for the whole time you held the car in the year. Its business distance is a reasonable estimate, which must take into account your logbook, your odometer records and any variations in the pattern of use of the car (section 28-90). You write the estimate and the percentage down before you lodge (section 28-100).

Broadly representative, and sometimes longer

The word representative is the ATO's, not the Act's. The ATO's Logbook method page says a logbook must "cover at least 12 continuous weeks and be broadly representative of your travel". If your circumstances change and the logbook "is no longer representative of your work-related use, you will need to complete a new 12 week logbook". The ATO's page on documents to keep adds: "A logbook may have to be kept for longer than 12 weeks if there is no regular pattern of use of the car."

A whole year counts too. The Act sets no longest period, and the ATO's example for question D1 keeps a logbook through the whole income year and works out its percentage straight from it.

Why not your best twelve weeks

Your records hold many overlapping 12-week stretches. The one with the highest business share is the highest your records allow, not a typical one, and claiming it overstates the year. An average of the highest and the lowest is no better: it can land either side of the truth, and neither end was typical. RyeDC still shows your best and least twelve weeks, marked for comparison only, not for your return.

How RyeDC finds it

Logbook periods, on the Report screen or behind Find your representative period on the Mileage screen, uses an equivalence test that compares each period with the rest of your year, rather than with the year that contains it. The representative period is the shortest one that matches the rest of your year.

It also uses change-point detection to look for the week your pattern of use changed. When it finds one, it names the week. A period across a change is never representative, and each side of the change gets its own period. Shifts and business drives are counted the same way the logbook counts them, so a period's percentage is the one its logbook prints.

Your estimate for the year

The screen's first card is your whole year, with the logbook estimate for the year: the representative period's percentage applied to the car's odometer distance for the year. If your pattern of use changed, each side takes its own period's percentage. That is the estimate to write down before you lodge, and the logbook prints it on its first page.

While the year runs, the card also projects the year's business use from your first week, with a ratio estimate of business to total distance and a range. With only a few weeks on record it is marked early, because the answer may still move.

If you recorded the whole income year with an odometer reading at each end, the whole year is itself a logbook and its share is your percentage. Once the year is over, RyeDC shows it first. The ATO says a logbook is valid for five years unless your circumstances change; if your pattern of use changed during the year, a period after the change is the one for the years ahead. What you cannot do is claim a year's share with no logbook behind it: the first year you use the logbook method for a car needs one.

When to keep the logbook longer

When no period matches the rest of your year, the screen says No representative period yet and what to do: keep recording, keep the logbook running longer, or wait for a period after a change in your pattern of use.

Recording it

Drives are recorded automatically, or logged by shift from the dash, and any period opens as the car logbook the ATO describes. The Help's Logbook periods covers the screen in full.

Sources

• Income Tax Assessment Act 1997, sections 28-90, 28-100, 28-115 and 28-120. ATO legal database, read 10 October 2026. ato.gov.au/law/view/document?docid=PAC/19970038/28-90
• ATO, Logbook method (individuals), QC 107247, last updated 12 June 2026, read 10 October 2026. ato.gov.au › Expenses for a car you own or lease › Logbook method
• ATO, Documents to support and verify your claims, QC 24929, last updated 29 July 2026, read 10 October 2026. ato.gov.au › Records you need to keep › Documents to support and verify your claims
• ATO, D1 Work-related car expenses 2026, QC 106604, last updated 30 May 2026, read 10 October 2026. ato.gov.au › Individual tax return 2026 › D1 Work-related car expenses 2026