Will your mileage log survive an IRS audit?

#What an examiner is actually checking

Vehicle expenses sit in a category the tax code treats more strictly than most. The consequence is practical: an examiner does not weigh whether your claim seems plausible. They check whether the record contains the required elements, and whether it looks like it was kept when the driving happened.

That is a much easier test to pass than people fear, and a much easier one to fail on a technicality. Everything below is from Publication 463, quoted rather than paraphrased, because the wording is the whole point.

#The elements, quoted

Table 5.1 of Publication 463 sets out what a record must show. For transportation, the row reads:

Cost of each separate expense. For car expenses, the cost of the car and any improvements, the date you started using it for business, the mileage for each business use, and the total miles for the year. ... Date of the expense. For car expenses, the date of the use of the car. ... Your business destination. ... Business purpose for the expense.

Four things per trip, then, plus one thing per year:

Element	What satisfies it	What does not
Date of the use	The actual date of the trip	A week or a month
Business destination	A named place. "Acme Ltd, Reading"	"Client"
Business purpose	"Site survey before quoting"	"Work", "business", or blank
Mileage for that use	14.3 miles, measured	"About 15"
Total miles for the year	Odometer on 1 January and 31 December	Nothing at all, which is the usual answer

That last row is the one most logs miss completely. Without a yearly total there is no denominator, so the share of the car used for business cannot be shown, and an examiner has nothing to test your business miles against.

#Six things that get a log rejected

1. Round numbers

Publication 463 says it in one line, under How To Prove Expenses: you cannot deduct amounts that you approximate or estimate. A column reading 20, 15, 30, 25, 20 down the page is an estimate wearing a spreadsheet. Real driving produces 14.3 and 31.8 and 6.2. Measured distances are the cheapest credibility you will ever buy.

2. A log with no business purpose

The publication is explicit that the purpose is normally written down:

You must generally provide a written statement of the business purpose of an expense. However, the degree of proof varies according to the circumstances in each case. If the business purpose of an expense is clear from the surrounding circumstances, then you don't need to give a written explanation.

The exception it gives is narrow and specific: a sales representative calling on customers along an established route does not have to explain the route every time. Unless your work looks like that, write the purpose. A log full of distances with an empty purpose column is half a record, and it is the half that proves the miles were business at all.

3. A whole year written in one sitting

This is the rule that decides most disputes:

You should record the elements of an expense or of a business use at or near the time of the expense or use and support it with sufficient documentary evidence. A timely kept record has more value than a statement prepared later when there is generally a lack of accurate recall.

Note what it does not say. It does not say you must write at every stop. Publication 463 explicitly allows a weekly habit: a log maintained on a weekly basis that accounts for use during the week is a timely kept record. Friday evening is enough. April is not.

4. Reconstruction with nothing behind it

Reconstruction has a defined place in the rules, and it is narrower than the advice you will read elsewhere:

If you can't produce a receipt because of reasons beyond your control, you can prove a deduction by reconstructing your records or expenses. Reasons beyond your control include fire, flood, and other casualties.

Losing a phone, cancelling a subscription, or simply not getting round to it are not casualties. That does not leave you with nothing, but it does mean you are in the incomplete records rule below rather than in the reconstruction rule, and the evidential burden is different.

5. Commuting counted as business

The trip from home to your regular place of work is personal, whatever is in the back of the car. Where your home is your principal place of business the first trip of the day can be a business mile, which is a genuinely valuable distinction, but it is also a common place for a log to overstate itself. If every single day starts with an identical business mileage figure from the same address, expect that to be the first question.

6. Two numbers on the return that disagree

The mileage claimed on the return has to reconcile with the log, and the log has to reconcile with the odometer readings. Where a year spans two rates, as 2026 does, one rate applied across the whole year produces a total that cannot be reproduced from the log. The correct treatment is 72.5 cents for trips to 30 June and 76 cents from 1 July, with the full working on the 2026 rate page.

#What the rules allow that almost nobody uses

The incomplete records rule

Where you lack complete records for an element, Publication 463 gives a route:

If you don't have complete records to prove an element of an expense, then you must prove the element with: Your own written or oral statement containing specific information about the element, and Other supporting evidence that is sufficient to establish the element.

And for business purpose specifically, the evidence is allowed to be indirect:

For example, the nature of your work, such as making deliveries, provides circumstantial evidence of the use of your car for business purposes. Invoices of deliveries establish when you used the car for business.

Job sheets, invoices, calendar entries, delivery history and service receipts with odometer readings on them are all evidence of this kind. They are not a substitute for a log. They are what makes a partial log hold.

Sampling

The most useful paragraph in the chapter, and the least known:

You can keep an adequate record for parts of a tax year and use that record to prove the amount of business or investment use for the entire year. You must demonstrate by other evidence that the periods for which an adequate record is kept are representative of the use throughout the tax year.

The publication's own worked example keeps proper records for the first week of every month, shows 75 percent business use, and uses invoices and bills to demonstrate the remaining weeks ran at the same rate. The burden is on the representativeness, not on the volume of paper. A sample taken in an unusual week proves the wrong thing, in either direction.

#Grade your own log

Ten questions. Count the yes answers.

1. Does every entry have a real date, and do the dates run in order?
2. Does every entry name a destination a stranger could find on a map?
3. Does every entry state a purpose in words, not a category?
4. Are the distances uneven and specific rather than round?
5. Do you have an odometer reading with a date on it near the start of the year?
6. And another near the end of the year?
7. Was each entry written within a week of the trip?
8. Are commuting trips excluded, or marked personal?
9. For 2026, are trips before and after 1 July valued at their own rate?
10. Does something outside the log corroborate a sample of it: an invoice, a calendar entry, a service receipt?

Nine or ten: your log is doing its job. Keep it and stop worrying. Six to eight: fixable, and the gaps are usually purpose and the year end odometer. Fix them going forward and gather corroboration for what is already written. Five or fewer: treat this year as a rebuild. Start an adequate log today, read the incomplete records rule above, and assemble the evidence that establishes the months behind you. The free log template has every column in this checklist, and what the IRS requires in a mileage log goes through each element in detail.

#How long you have to keep it

Generally, this means you must keep records that support your deduction (or an item of income) for 3 years from the date you file the income tax return on which the deduction is claimed. A return filed early is considered filed on the due date.

There is a longer obligation attached to the car itself where you claim depreciation: you must keep records of the business use of your car for each year of the recovery period. In practice that means the log outlives the three year window, so keep it with the vehicle paperwork rather than with the year's receipts.

#If the letter arrives

Publication 463 sets the expectation plainly:

If your return is examined, you may have to provide additional information to the IRS. This information could be needed to clarify or to establish the accuracy or reliability of information contained in your records, statements, testimony, or documentary evidence before a deduction is allowed.

So the log is the opening document, not the whole case. Send it with the corroboration attached rather than waiting to be asked: the odometer photographs, a handful of invoices matching logged trips, the calendar for a sampled month. A log that arrives with its own evidence reads as a record. A log that arrives alone reads as a claim.

And if none of this exists yet because months went unlogged, the honest path through it is set out in what to do when you forgot to track your miles.