To record mileage for tax purposes, write down four things for every business trip: the date, the number of miles driven, where you went, and the business reason for the trip. Add two more numbers once a year: your vehicle’s odometer reading on January 1 and December 31. Those six pieces of information, kept consistently throughout the year, are what the IRS regulation on vehicle expense substantiation actually requires.1eCFR. 26 CFR 1.274-5 – Substantiation Requirements
The Four Things Every Trip Entry Needs
Federal law requires “adequate records” for any vehicle expense deduction.2Office of the Law Revision Counsel. 26 U.S. Code 274 – Disallowance of Certain Entertainment, Etc., Expenses The Treasury regulation interpreting that statute translates “adequate” into four specific fields per trip:
- Date. The specific date of the trip. Weekly or monthly summaries don’t satisfy the rule.
- Miles driven. The number of business miles for that trip.
- Destination. Where you went. A client name, an office address, or a city is enough.
- Business purpose. Why the trip was business-related. Short descriptions like “client meeting,” “supply pickup,” or “site inspection” work.
Each entry needs all four. A log that lists dates and miles but no destinations, or destinations but no purposes, is incomplete and can be thrown out during an audit even if the underlying trips were real.
Odometer Readings at the Start and End of the Year
Beyond individual trip entries, you have to record the vehicle’s odometer at the beginning and end of each tax year.1eCFR. 26 CFR 1.274-5 – Substantiation Requirements Those two numbers set your total miles for the year. Broken down against your business, commuting, and personal miles, they let the IRS check the ratio of business use to total use. If those figures don’t reconcile, the whole deduction is exposed.
Before the year closes, add up your entries into annual totals for business, commuting, and personal miles, then compare that sum against the January 1 and December 31 readings. A discrepancy caught in December is easy to fix. The same discrepancy surfacing in an audit is not.
Record It the Same Day, Not Later
Write each trip down at or near the time it happens. The IRS calls this “contemporaneous” record-keeping, and logs put together weeks or months after the fact carry far less weight in an audit. A log reconstructed from memory after an audit notice arrives is close to worthless.
Contemporaneous doesn’t mean the moment you park. Same-day, or within a few days, is what auditors expect to see. The pattern the IRS looks for is regular entries made throughout the year, not a burst of activity right before filing.
Paper, Spreadsheet, or App
The IRS doesn’t specify a format. What it requires is that the record be legible, organized, and complete on all four trip fields plus the odometer readings. Three formats all work:
- Paper logbook. Pre-printed mileage logs from office supply stores are fine. Fill them in the same day. The weakness is that paper gets lost and there’s no backup unless you photograph the pages.
- Spreadsheet. Columns for date, starting odometer, ending odometer, destination, and purpose give you year-end totals and a searchable record. Keep backups.
- GPS-based apps. Mileage apps record routes automatically using your phone’s GPS and let you classify each trip as business or personal. These generally produce the most audit-resistant logs, because the underlying data includes timestamps, route maps, and distances that are harder to dispute than handwritten entries.
Whatever format you choose, use it consistently. Switching methods mid-year is fine as long as no trips fall through the gap.
Which Trips You’re Actually Allowed to Log
A perfect log of the wrong trips still fails. The classification rules matter as much as the record-keeping itself.
The core rule: driving between your home and your regular workplace is commuting, and commuting is never deductible, no matter how long the drive.3Internal Revenue Service. Publication 463 – Travel, Gift, and Car Expenses That holds even if you take business calls during the drive or run promotional graphics on the vehicle.
Trips that generally do count as business miles include travel between your main office and a secondary work location, visits to client sites, drives to pick up supplies, and travel between two business locations during the workday. Trips to a temporary work location, meaning one where you realistically expect to work for a year or less, are also deductible even if you leave from home, as long as you have at least one regular place of work.3Internal Revenue Service. Publication 463 – Travel, Gift, and Car Expenses If your expectation changes and you realize the assignment will last longer than a year, the deduction stops on the date that expectation shifts.4Internal Revenue Service. Topic No. 511, Business Travel Expenses
Home Office Changes the Answer
If you have a qualifying home office that serves as your principal place of business, the commuting rule flips. Trips from that home office to any other work location in the same trade or business become deductible business miles.3Internal Revenue Service. Publication 463 – Travel, Gift, and Car Expenses A freelance consultant leaving a qualifying home office to visit a client can log the entire round trip. Without the qualifying home office, that same drive is non-deductible commuting.
No Regular Office
If you don’t have a regular workplace but generally work within your metropolitan area, you can deduct travel to temporary work sites outside that metro area. Trips to temporary sites inside the metro area are commuting and don’t count.3Internal Revenue Service. Publication 463 – Travel, Gift, and Car Expenses
Parking fees and tolls tied to business trips are deductible separately, on top of the mileage figure, so log those receipts alongside the trip they belong to.5Internal Revenue Service. Topic No. 510, Business Use of Car
How Long to Keep the Log
Keep records for at least three years from the date you filed the return, or two years from the date you paid the tax, whichever is later.6Internal Revenue Service. How Long Should I Keep Records That window matches the standard period during which the IRS can assess additional tax.7Internal Revenue Service. Topic No. 305, Recordkeeping If you underreported income by more than 25%, the retention period extends to six years. Digital backups make the storage effortless.
What Happens If Your Log Doesn’t Hold Up
If you claim a mileage deduction and can’t substantiate it, the IRS will disallow the deduction entirely. Losing the deduction means owing the tax you thought you’d avoided, plus interest.
On top of that, the IRS can add a 20% accuracy-related penalty on the resulting underpayment if it finds you were negligent, meaning you didn’t make a reasonable attempt to comply.8Office of the Law Revision Counsel. 26 U.S. Code 6662 – Imposition of Accuracy-Related Penalty on Underpayments Sloppy records or missing records can qualify.
If the IRS concludes a log was fabricated or business miles knowingly inflated, the civil fraud penalty is 75% of the underpayment attributable to fraud, and the entire underpayment is treated as fraudulent unless the taxpayer proves otherwise.9Office of the Law Revision Counsel. 26 U.S. Code 6663 – Imposition of Fraud Penalty The gap between “I forgot to keep a log” and “I invented one” is the difference between a 20% penalty and a 75% penalty. A consistent, contemporaneous log built entry by entry through the year is the cleanest protection against both.