Why mileage gets special scrutiny
Most business expenses just need to be “ordinary and necessary.” Vehicle use is different: it falls under the strict substantiation rules of Internal Revenue Code §274(d), the same regime as travel and meals. That means no log, no deduction— an auditor can (and will) disallow car expenses entirely if the records aren't there, even when the driving obviously happened. Courts regularly refuse to estimate mileage for taxpayers, because for listed property like cars, the law forbids it.
The four things every trip needs
Per IRS Publication 463, each business trip in your log should record:
- Date — when you drove.
- Destination— where you went (city or place is enough; “client office, Riverside” beats an exact address you'll never need).
- Business purpose— why the trip was business: “delivery block,” “showing at 14 Elm St,” “supply run — packaging.”
- Miles — the distance for that trip.
On top of per-trip records, your tax return asks for yearly totals: total miles driven, business miles, and commuting miles (Schedule C asks these directly, along with whether you have written evidence). Noting your odometer on January 1 and December 31 makes those totals trivial and corroborates the log.
“Contemporaneous” — the word that decides audits
The IRS wants records made at or near the time of the trip. You don't need to write while parked at the curb — Publication 463 accepts records updated weeklyas “made at or near the time.” What fails is the log produced in March for the previous year, in one sitting, in one pen. Auditors have seen thousands of those; uniform handwriting, round numbers, and no variance are exactly what they look for.
If you're already behind, don't invent — there is a defensible way to reconstruct a log. But the real fix is a habit that takes seconds per trip, which is precisely the job of a mileage app.
What formats count
Anything, honestly: a paper notebook, a spreadsheet, an app. The IRS cares about the content and timingof the record, not the medium. An account book, diary, log, trip sheet, or “similar record” all qualify. An app has two practical advantages: it timestamps entries as you go (contemporaneous by construction), and it does the math — applying the correct standard mileage rate for the date and producing a clean total your preparer can drop into the return.
Common mistakes that sink logs
- Counting commuting. Driving from home to a regular workplace is personal, no matter how far. (Trips from a qualifying home office to work sites are different — that nuance matters for home-based businesses.)
- Round-number estimates.“About 200 miles a week” is not a record; it's an invitation to disallowance.
- Missing the purpose. A list of dates and miles with no reason for each trip fails the statute.
- No total-miles denominator. Business-use percentage needs total annual miles too; keep those year-end odometer readings.
- Tossing the log after filing. Keep records at least 3 years from filing (longer never hurts).
A trip in a compliant log, concretely
Mar 14, 2026 — Home office → Ace Restaurant Supply, Portland — pick up packaging stock — 18.4 mi — business.
That's the whole standard. Date, place, purpose, miles, recorded that week. Do that for every business trip and your deduction is about as audit-proof as a deduction gets.