Yes — if you plan to claim any vehicle-related deduction on your taxes, you need an odometer reading. The IRS expects two of them for each vehicle: the reading on the first day you use the car for a deductible purpose that year, and the reading on the last. Those two numbers, paired with a per-trip log of your deductible drives, are what separates a defensible deduction from one that gets thrown out. This is true whether you use the standard mileage rate or the actual expense method, and it is true for business, medical, and charitable driving alike.
What the IRS Wants From Your Odometer
The substantiation rule under Section 274(d) is direct: no adequate records, no deduction.1Office of the Law Revision Counsel. 26 USC 274 – Disallowance of Certain Entertainment, Etc., Expenses To satisfy it, you need documentation on two levels.
The first is your annual odometer readings. Record the number on January 1 and again on December 31, or on the dates you start and stop using the vehicle for a deductible purpose during the year. These two figures establish total miles driven, which is the denominator for calculating your business-use percentage.
The second is a per-trip log. For every deductible drive, the IRS wants four pieces of information:2Internal Revenue Service. Publication 463, Travel, Gift, and Car Expenses
- The date of the trip.
- The destination.
- The business, medical, or charitable purpose (for example, “met with client about project scope” or “drove to hospital for physical therapy”).
- The mileage — either start and end odometer readings or total trip miles.
Entries must be made at or near the time of each trip. A log reconstructed from memory at tax time, or a lump-sum annual estimate, is exactly the kind of record that gets rejected on examination.3eCFR. 26 CFR 1.274-5 – Substantiation Requirements The regulation specifically calls for “an account book, diary, log, statement of expense, trip sheet, or similar record” maintained contemporaneously.
Why the Annual Readings Matter
Under the standard mileage rate, you multiply your qualifying miles by the IRS rate — 72.5 cents per mile for business in 2026, 20.5 cents for medical, and 14 cents for charitable.4Internal Revenue Service. IRS Sets 2026 Business Standard Mileage Rate at 72.5 Cents Per Mile Even here, where the total-miles figure does not directly change the math, an odometer reading anchors your log and shows an examiner that the deductible miles are a plausible share of the vehicle’s overall use.
Under the actual expense method, the annual readings do direct work. You total every operating cost — fuel, insurance, repairs, tires, registration, lease payments, depreciation — and multiply by your business-use percentage. If you drove 20,000 total miles and 15,000 were for business, that percentage is 75%. Skip the January 1 reading and you cannot prove the denominator of that fraction. Depreciation caps, which for a passenger vehicle placed in service in 2026 start at $20,300 in year one with bonus depreciation and $12,300 without, also apply only to the business-use portion.5Internal Revenue Service. Rev. Proc. 2026-15 Your mileage records set the ceiling.
Acceptable Formats for Your Log
The IRS does not require any particular format. A paper notebook, a spreadsheet, or a mileage-tracking app all work, as long as the four required elements are present and you can produce the records for review.2Internal Revenue Service. Publication 463, Travel, Gift, and Car Expenses Many drivers find a GPS-based phone app the easiest way to capture trip data in real time, which also satisfies the contemporaneous requirement without depending on memory.
For the annual readings themselves, a dated photograph of the odometer works. So does a note in your mileage app, an entry in your accounting software, or a line at the top of a paper log. The point is that the number is captured on the day, not estimated later.
One boundary worth naming: regular W-2 employees generally cannot deduct unreimbursed vehicle expenses on their federal return at all, following the Tax Cuts and Jobs Act. Armed Forces reservists, qualified performing artists, and fee-basis state or local government officials remain exceptions and file Form 2106.6Internal Revenue Service. Topic No. 510, Business Use of Car If you are an employee outside those categories, your mileage records still matter — but for your employer’s reimbursement plan, not your 1040.
What Happens Without the Records
If the IRS challenges your vehicle deduction and you cannot produce adequate contemporaneous records, the deduction is disallowed. There is no partial credit for trying. Section 274(d) treats the substantiation requirement as a threshold: either you meet it or the deduction goes away.
On top of the lost deduction, you may face an accuracy-related penalty of 20% of the resulting underpayment if the IRS finds negligence or a substantial understatement of tax.7Internal Revenue Service. Accuracy-Related Penalty For a self-employed taxpayer claiming $7,000 in business mileage deductions in a 22% bracket, the arithmetic is roughly $1,540 in lost tax savings plus another $308 in penalties. All for not keeping a log.
How Long to Keep Your Mileage Records
The default retention period is three years from the date you filed the return. If you underreported gross income by more than 25%, the IRS has six years to assess additional tax, and your records need to last that long as well.8Internal Revenue Service. How Long Should I Keep Records?
For a vehicle you are depreciating under the actual expense method, keep the records for three years after the final year of depreciation, or three years after you sell the vehicle, whichever is later. When you dispose of a depreciated vehicle, the IRS requires you to recapture prior depreciation as ordinary income, and your mileage history is what makes that calculation possible.
A Two-Second Habit That Protects the Whole Year
A photo of your odometer on January 1 takes seconds and anchors an entire year of documentation. Pair it with a per-trip log kept as you drive, save both through the retention window, and the odometer question stops being a question. You have what the IRS asks for, in the form the regulation names, on the day the trip happened.