Mileage deductions are rarely disallowed because the driving was not deductible. They are disallowed because the taxpayer could not prove it happened.
The standard is higher than for most expenses. Vehicle expenses fall under a stricter substantiation rule, which means a credit card statement and a good-faith estimate are not enough. Here is what a log needs to contain, and what gets people into trouble.
The four elements
For every business trip, you need:
- Date of the trip
- Mileage for that trip
- Destination — where you went
- Business purpose — why you went
Plus one thing at the year level: total miles driven on the vehicle for the year, so that business use as a percentage of total use can be computed. Most incomplete logs are missing this one.
“Contemporaneous” is the word that matters
Records should be made at or near the time of the trip. A log built in March from calendar entries and memory is a reconstruction, and it carries much less weight than one written as you went.
Reconstructions are not automatically worthless — corroborating evidence like appointment calendars, client invoices, and service records showing odometer readings can support one. But you are arguing rather than proving, and the outcome is far less certain.
The practical rule: record within a few days, not a few months.
What a good entry looks like
| Date | Destination | Purpose | Miles |
|---|---|---|---|
| 3/14 | Henderson residence, Toms River | Estimate — bathroom remodel | 23 |
| 3/14 | Riverside Supply, Brick | Pick up fixtures, Henderson job | 11 |
| 3/15 | Miller job site, Lakewood | Install day 2 | 31 |
Compare that to what actually shows up in most logs: “3/14 — business — 34 miles.” The second version tells an examiner nothing and looks like it was written all at once.
The purpose field is where logs live or die. “Client meeting” is thin. “Estimate — bathroom remodel, Henderson” ties the trip to a specific job that presumably also appears in your invoices. That cross-reference is what makes a log credible.
Red flags that draw attention
- 100% business use. Possible, but rare, and it invites the question of how you get groceries. If you have a second personal vehicle, that fact supports the claim — note it.
- Suspiciously round numbers. A log of 15,000 business miles composed entirely of 50s and 100s reads as estimated.
- Identical mileage every day. Real driving varies.
- Business miles exceeding total miles, or totals that do not reconcile with odometer readings on service records. This is a quick and common catch.
- A log in one handwriting, one pen, all at once. Contemporaneous records look like they were made over time.
Apps versus spreadsheets versus paper
Apps are the strongest option for most people. GPS-tracked trips are automatically dated and measured, timestamps are inherently contemporaneous, and classification takes a swipe. The weakness is the purpose field — apps let you categorize as “business” without saying why. Fill in the note field. An app log with no purposes is a strong record of driving and a weak record of deductibility.
Spreadsheets work if you are disciplined. The risk is batching — sitting down monthly to fill it in from memory, which quietly makes it a reconstruction. Enter weekly at worst.
Paper is fine and still holds up. A notebook in the glovebox filled in at each stop is genuinely contemporaneous. Keep it, and photograph pages periodically in case it is lost.
The odometer readings worth capturing
Record the odometer on January 1 and December 31. That single pair establishes total miles for the year, which is what lets you compute business percentage — and it is the number missing from most otherwise decent logs.
Also keep service records. Oil change invoices show dated odometer readings, and they independently corroborate your totals. That third-party trail is disproportionately useful.
The sampling method
There is a narrower option: keeping a log for a representative portion of the year and extrapolating. It requires that the sample genuinely represents the full year, and it works best for driving patterns that are highly consistent week to week.
It is legitimate but harder to defend, and it fails badly for seasonal businesses — a landscaper’s April is not a representative sample of their January. Full-year logging is safer, and modern apps have made the effort argument largely obsolete.
How long to keep it
Keep mileage logs at least three years from the filing date, matching the general assessment period. If a vehicle is depreciated under the actual expense method, keep records for that vehicle for the entire depreciation period plus three years — you may need to substantiate business use percentage across all of it, and disposition can trigger recapture that reaches back.
Digital copies are acceptable. Photograph paper logs.
Frequently asked questions
Can I use my calendar as a mileage log?
Not on its own, since it lacks mileage. As corroboration for a reconstruction it helps considerably.
What if I lost my log?
Reconstruct from calendars, invoices, client records, and service receipts with odometer readings. It is weaker than a contemporaneous log, but “no records” and “imperfect records” are different outcomes.
Do I need a log if I use actual expenses instead of standard mileage?
Yes. You still need business-use percentage to prorate every vehicle cost, and that requires the same trip records.
Does a GPS app’s data alone satisfy the requirement?
It covers date, distance, and destination well. It does not cover business purpose unless you enter it. Add the note.
What about a vehicle used by employees?
Same substantiation applies, and personal use of a company vehicle generally becomes taxable compensation. That is a payroll issue as much as a deduction issue.
The bottom line
Date, destination, purpose, miles — every trip — plus year-end odometer readings. Record it within days, write purposes that tie to real jobs or clients, and keep the service records that corroborate your totals.
The deduction is usually not the hard part. The proof is.
Related guides
- Business miles vs. commuting miles
- The 2026 mid-year mileage rate change
- 2026 mileage deduction calculator
- Parking and tolls deduction
This article is general information, not tax advice. Substantiation requirements come from IRS Publication 463 and Section 274(d); verify current guidance at irs.gov or consult a qualified tax professional.