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Reconstructing a mileage log after missing months
Honest guidance when the log has gaps: what evidence helps, what the CRA accepts and does not, and why prevention is cheaper than reconstruction.
It is October. You run a one-person renovation business in Hamilton, Ontario, and you meant to keep a log. You have a few weeks from January, nothing from February to August, and an accountant asking for a business-use percentage. This post is about what you can honestly do from here, and what you cannot.
The gap has a price. Say the car cost $9,000 to run this year and about 45 percent of its driving was for jobs: that is $9,000 × 45% = $4,050 on your T2125. If the reconstruction only supports 30 percent, the claim is $9,000 × 30% = $2,700, and $1,350 of deduction is gone.
What the CRA expects, and why gaps matter
The CRA asks for a record of each business trip with its date, destination, purpose and distance, made at or near the time of the trip. The phrase that matters is “at or near the time.” A log written in October about March is not that. It can still be evidence, but it is weaker evidence, and an auditor is entitled to discount it.
The same is true in the United States. IRS guidance on business use of a car calls for adequate records, and a record made contemporaneously carries far more weight than one assembled later.
So the goal of reconstruction is not to produce a log that looks like it was kept all year. It is to produce a defensible estimate, clearly labelled as such, built from documents that do exist.
The sources you probably have
Start with anything that has a date and a place.
Your calendar. Client meetings, site visits, deliveries, court dates, open houses. Each entry gives you a date, a destination and a purpose. What it does not give you is the distance, so you look that up: home or office to the address, by the route you would normally drive.
Invoices and receipts you issued. An invoice to a client at their address on a given date is strong support that you went there. Service tickets, delivery confirmations and signed work orders do the same.
Receipts you received. Fuel, parking and toll receipts place you at a location on a date. Parking near a client’s office on a day you invoiced them is a solid pairing.
Vehicle service records. An oil change receipt usually records the odometer. Two service receipts four months apart give you the total kilometres driven in that window, which is a ceiling on how much business driving could have happened.
Odometer photos. Some people photograph the dash at fill-ups or on the first of the month without thinking about it. Check your camera roll.
Phone location history, if you kept it. Some map apps keep a timeline. It is a useful cross-check for dates and places, though it does not know why you went.
Recurring patterns. If you visit the same three job sites every Tuesday and Thursday, and your invoices confirm the work continued, a pattern-based reconstruction is reasonable for the days you cannot otherwise document. Write down the reasoning.
How to assemble it
Build a table with the same four columns as a proper log: date, destination, purpose, distance (the mileage log template has them). Add a fifth column for the source: “calendar,” “invoice 2026-0412,” “service receipt,” “pattern.” That column is what turns a list into evidence. It also keeps you honest, because a row with nothing in the source column is a guess, and you should remove it or flag it.
Then sanity-check the total against the odometer. If service receipts show the car travelled 9,000 km between March and August, and your reconstructed business trips sum to 8,700 km, you have almost no personal driving for six months. That is not believable for most people, and an auditor will say so. Reduce the estimate or find the error. The same check, done monthly, is in odometer readings for a CRA mileage log.
The months still ahead do not need rebuilding. Odie starts and ends trips without being opened, records the date, endpoints, route and distance, and asks Work or Personal from the lock screen, so November and December arrive as a log rather than a project. Here is how Odie keeps a self-employed mileage log.
What the CRA accepts, and what it does not
The CRA does not publish a rule that says a reconstructed log is rejected. It says records must be adequate to support the claim. In practice:
- A reconstruction backed by third-party documents (invoices, receipts, service records) is taken seriously, though it may be discounted.
- A reconstruction that is purely from memory, with round numbers and no supporting paper, is close to worthless.
- A reconstruction presented as if it were contemporaneous is worse than a gap, because it damages the credibility of everything else you filed.
If the gap is large and the evidence is thin, the honest options are a lower claim or, for the self-employed, a conservative business-use percentage your accountant is comfortable defending. A smaller number that survives review is worth more than a larger one that does not.
Note that the three-month simplified logbook is not a rescue for a missing year. It requires a full base year first, and the sample has to be a log you actually kept during those three months.
Tell your accountant
Do not hand over a reconstruction without saying what it is. Your accountant will decide how to present it and may want to reduce the claim. They will also know whether your situation has any flexibility the rules allow. What they cannot do is defend a number they were told was contemporaneous when it was not.
Why prevention is cheaper
Reconstructing six months takes an evening or a weekend, produces a weaker record, and usually ends with a lower claim. Keeping the log as you go takes a few seconds per trip. The difference in effort is not close, and the difference in value at audit time is large.
The most common reason logs lapse is that they depend on remembering to start something at the beginning of each drive. The fix is to move the recording somewhere that does not depend on memory: a habit tied to parking, a notebook clipped to the visor, or an app that starts by itself.
Odie’s log also covers its own gaps. Trips it missed can be added by hand with an address search and a driving distance from Apple Maps, and dated odometer readings show what share of the car’s total driving has been logged, so a gap shows up as a number rather than as a surprise in October. The sample CRA year report (PDF) shows that share next to the odometer bounds and the trip log.
Questions
Will the CRA accept a reconstructed mileage log?
The CRA does not publish a rule that rejects one; it says records must be adequate to support the claim. A reconstruction backed by invoices, receipts and service records is taken seriously though it may be discounted, while one from memory with round numbers is close to worthless.
What records can I use to rebuild a mileage log?
Anything with a date and a place: your calendar, invoices you issued, fuel, parking and toll receipts, service records with odometer readings, and odometer photos. Add a source column so every row points to the document behind it.
Can the three-month sample logbook cover missing months?
No. The simplified logbook requires a full base year first, and the three-month sample has to be a log you actually kept during those months.
Should I tell my accountant the log was reconstructed?
Yes. A reconstruction presented as if it were kept at the time is worse than a gap, and your accountant cannot defend a number they were told was contemporaneous when it was not.
Sources
Every figure above is checked against the sources listed. How we check our facts. Not tax advice; your accountant knows your situation. Spotted an error? Tell us.
