The mileage deduction has an unusual weakness: it lives or dies on paperwork. Car expenses fall under the tax code's strict substantiation rules (§274(d)), which means an auditor doesn't have to estimate or give you the benefit of the doubt. No qualifying log, no deduction — even when the miles were real. Tax Court cases affirm this pattern year after year: taxpayers with genuine business driving lose the entire deduction because the record-keeping failed.
The rule in one sentence: for every business trip, record the date, the miles, where you went, and why — at or near the time you drove it — and keep evidence of your total annual mileage (start-of-year odometer readings).
The four fields every trip needs
IRS Publication 463 spells out the elements you must be able to prove for each business use of your car:
- Date — when the trip happened.
- Mileage — the distance of the trip (your evidence for the year's total business miles).
- Destination — where you went (city, place, or client site).
- Business purpose — why the trip was business. "Meeting with Rivera Construction re: kitchen bid" survives; "errands" doesn't.
Vague entries are the most common self-inflicted wound. An auditor reading "client visit, client visit, client visit" with no names has every reason to dig; specific, boring detail is what credibility looks like.
"Contemporaneous": the timing requirement
You don't need to write the log while parked outside the client's office — but you can't invent it in April either. Publication 463 requires records made at or near the time of the expense, and explicitly blesses a log kept on a weekly basis as timely. A statement prepared months later "generally has less value," and logs reconstructed after an audit notice arrives are routinely rejected outright.
This is the requirement that quietly kills most DIY systems. A notebook in the glovebox works only if you actually write in it every drive; a spreadsheet works only if you remember Tuesday's trips on Sunday night. An automatic tracker sidesteps the problem — the record is created during the drive, which is as contemporaneous as it gets.
Odometer readings: the anchor auditors ask for
Your per-trip log proves which miles were business. Odometer readings prove the total pool of miles those trips came out of. Schedule C asks directly for your vehicle's total, business, commuting, and other miles for the year — so record each car's odometer on January 1 (and when you start using a car for business mid-year). Repair invoices and inspection reports, which note the odometer, quietly corroborate those readings; auditors cross-check them.
Total miles matter for a second reason: the business-use percentage they establish is what splits actual expenses (and supports things like the business share of loan interest) if you ever use that method instead of the standard rate.
Why bank statements and gas receipts aren't enough
A credit-card statement full of gas purchases proves you bought gas — not where you drove, how far, or why. Under the standard mileage rate the deduction isn't even based on fuel spending, so receipts don't substantiate anything by themselves. The IRS wants the log; receipts are, at best, supporting color. (The reverse trap exists too: fuel spending wildly out of proportion to claimed mileage is an audit flag.)
What surviving an audit actually looks like
If your return is examined, the examiner will typically want:
- The mileage log itself — every trip, with the four fields, kept timely;
- Start/end-of-year odometer evidence for each vehicle;
- Corroboration that the trips were real: calendar entries, client appointments, invoices, delivery records, repair receipts with odometer readings;
- Consistency — the log's totals should match what's on the return.
A digital log from an app satisfies the rules exactly as a paper one does — the IRS cares that records are timely, complete, and credible, not what they're written on. Publication 463 even allows substantiating a full year from an adequate sample (for example, a complete first-week-of-each-month log) when you can show the sample is representative — a useful fallback, but a full log is always stronger.
What's at stake
A contractor claiming 15,000 business miles in 2026 is deducting $10,875 (15,000 × 72.5¢). If the log doesn't hold up, that entire amount can be added back to taxable income — plus interest and, often, accuracy-related penalties. The log is the deduction.
The compliant-log checklist
- ☐ Every business trip has date, miles, destination, and a specific purpose
- ☐ Entries are made during the drive, same day, or at worst weekly
- ☐ January 1 odometer reading recorded for each vehicle, every year
- ☐ Year-end totals: total / business / commuting / other miles per vehicle
- ☐ Log totals match the return, and calendar/receipts corroborate the story
This is precisely the shape of record Mile produces automatically: drives are captured as they happen with route, distance, and time; you tag the purpose with a swipe; each vehicle stores its January 1 odometer reading; and the CSV/PDF export lists every trip with the four fields, the IRS rates applied per year, and the odometer substantiation block your accountant wants.
Sources
- IRS, Publication 463 — Travel, Gift, and Car Expenses (Chapter 5: Recordkeeping — adequate records, sampling, and timely-kept records)
- IRS, Topic No. 510 — Business use of car
- IRS, Schedule C instructions — Part IV, Information on Your Vehicle
- 26 USC §274(d) — substantiation required for listed property, including vehicles