If you deduct vehicle costs on Schedule C, the IRS expects a mileage log. Here is exactly what your records have to show, why timing matters, and what is at stake in an audit if they are missing.
The rule is the same whether you use the standard mileage rate (72.5 cents per mile in 2026) or the actual expense method: you have to be able to prove your business miles. The IRS does not accept a single round number at tax time. It wants a record that shows how you arrived at the figure, trip by trip.
For each business drive, your log needs four things:
You should also record your vehicle's odometer reading at the start and end of the year. That fixes your total miles for the year, which is what business miles are measured against. If you ever use the actual expense method, the business-use percentage comes straight from those two numbers.
"Contemporaneous" is the word the IRS uses, and it carries weight. It means the record is made at or near the time of the trip, not pieced together later. A mileage figure rebuilt from old calendar invites and bank statements the week before you file is exactly what auditors are trained to discount.
This is the most common way self-employed people lose the deduction: the miles were real, but the record was not kept as they went. Logging each trip the day it happens, on paper or in an app that timestamps the entry, is what makes the number hold up.
Cars and trucks are treated as listed property under the tax code, which puts them under strict substantiation rules. In plain terms: the usual principle that lets a taxpayer estimate a reasonable business expense does not apply to vehicle use. No log, no deduction.
If you cannot produce adequate records, the IRS can disallow the entire mileage deduction, not just the trips it doubts. On a 6,000-mile year that is a $4,350 deduction erased, plus back tax, interest, and possible penalties. For a freelancer also paying 15.3% self-employment tax, the real cost of a missing log runs well past the deduction amount itself.
The IRS allows a sampling approach. You can keep a detailed log for a representative stretch of the year, a common choice is the first 90 days, and apply that business-use pattern to the full year. The catch is in the word representative. The sample only holds up if that period reflects how you normally drive.
If your work is seasonal, or if a slow quarter looks nothing like a busy one, a sample will misstate your real business use and the IRS can reject it. When your driving is steady, sampling saves effort. When it is not, a full-year log is the safer record.
Keep your mileage log for at least three years after you file the return that claims the deduction (or two years from the date you paid the tax, whichever is later). If you used the actual expense method and depreciated the vehicle, keep the records longer, since depreciation reaches back across multiple years.
Any of these satisfies the IRS, as long as it captures the four required details at the time of each trip:
The failure point for every manual method is the same: skipped entries. A few forgotten weeks quietly shrink your deduction, and you only notice at tax time when the miles will not add up.
Once your log is in order, the 2026 IRS mileage rate guide shows how to turn business miles into a Schedule C deduction, and the Self Employment Tax Estimator shows how that deduction lowers your quarterly tax bill.
Self Employment Toolkit timestamps every trip, records the four required details, and exports a complete mileage log for your accountant. Free to use.
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