What Does the IRS Actually Require You to Record in a Mileage Log?

The IRS imposes strict substantiation rules under Section 274(d) that apply to every vehicle expense deduction, whether you use the standard mileage rate or actual expenses. A compliant mileage log must capture four elements for each trip: the date, the destination, the business purpose, and the mileage. Records should be created at or near the time of each trip -- a standard the IRS calls "contemporaneous." When logs are missing or incomplete, the IRS has very limited flexibility to accept substitutes, and courts have generally upheld that limitation.

Why the mileage log rules are stricter than most recordkeeping rules

Most business expenses are governed by the general substantiation standard in the tax code, which gives taxpayers some room to reconstruct records or rely on estimates. Vehicle expenses are different. Section 274(d) of the Internal Revenue Code imposes a heightened substantiation requirement that explicitly overrides the general rules. No deduction or credit is allowed for listed property -- which includes passenger automobiles -- unless the taxpayer can prove the expense with adequate records or sufficient corroborating evidence.

This means the usual fallback of a reasonable estimate or a credible taxpayer statement is not enough on its own. Congress wrote the stricter standard into law specifically because vehicle expenses were a frequent area of abuse.

The four things every mileage log entry must show

Treasury Regulation 1.274-5T spells out what an "adequate record" must contain for each business trip:

Date of the trip
Destination -- the city or area, or the name of the customer or location visited
Business purpose -- a brief description of the business reason for the trip or the business benefit expected
Mileage -- the number of miles driven for that specific trip

Many practitioners also recommend recording the odometer reading at the start and end of each trip, or at least at the start and end of each day. While the regulation does not require per-trip odometer readings, having them makes it far easier to verify total annual business mileage and to separate business miles from personal miles.

The contemporaneous requirement

The IRS expects records to be created at or near the time of each trip. The regulation uses the word contemporaneous, and IRS guidance makes clear that a log reconstructed weeks or months later -- especially one prepared after an audit notice arrives -- carries much less weight than one maintained in real time.

A record is generally considered contemporaneous if it is made at the time of the expense or within a reasonable period afterward. In practice, most tax professionals treat "end of the same day" as the safe outer boundary for trip-by-trip entries.

Standard mileage rate vs. actual expenses: does the log requirement change?

No. Both methods require the same underlying mileage log. The standard mileage rate simply substitutes a flat per-mile amount for tracking fuel, oil, repairs, insurance, and depreciation separately. It does not eliminate the need to document each business trip. If you cannot show how many business miles you drove, you cannot use either method.

How flexible is the IRS when a mileage log is missing or incomplete?

This is where many taxpayers are surprised -- and not pleasantly. Because Section 274(d) explicitly overrides the general reconstruction rules, the IRS and the Tax Court have very little room to allow substitutes. Here is how the landscape actually looks:

The Cohan rule does not apply

Under the old Cohan v. Commissioner case, courts could estimate a deduction when a taxpayer had no records but there was credible evidence that some expense was incurred. Section 274(d) was enacted specifically to eliminate that flexibility for listed property including vehicles. Courts have repeatedly held that the Cohan rule cannot save a vehicle deduction when adequate records are absent.

Corroborating evidence can sometimes substitute for a formal log

The regulation does allow a second path: "other sufficient evidence corroborating the taxpayer's own statement." This is a narrow opening. To use it, a taxpayer generally needs both a written or oral statement explaining the business use and independent corroborating documents such as:

Appointment calendars or meeting records showing client visits on specific dates
GPS or fleet tracking data
Invoices or delivery records tied to specific trips
Employer reimbursement records that document destinations and purposes

The key word is independent. A taxpayer's after-the-fact reconstruction alone, without any supporting documents, rarely survives audit or litigation under the Section 274(d) standard.

Partial logs receive partial credit

If a mileage log covers only part of the year, the IRS may allow a deduction for the documented portion and disallow the rest. Some taxpayers have also used a representative sample period -- for example, a log kept for three months -- to establish a percentage of business use, then applied that percentage to the full year. The IRS accepts this approach only when the sample period is truly representative and the taxpayer can demonstrate consistent driving patterns throughout the year.

The IRS auditor's discretion is narrow

Revenue agents do have some discretion in how aggressively they pursue documentation, and some audits are resolved with less-than-perfect records when the overall return appears credible. However, that discretion is informal and not guaranteed. The statutory rule under Section 274(d) does not give auditors authority to simply waive the requirement. Any flexibility exercised at the audit level can be reversed on appeal or in Tax Court, which will apply the statute as written.

Practical recordkeeping tips

Use a dedicated mileage tracking app that records GPS data automatically -- this creates a contemporaneous, timestamped log that is difficult to challenge.
If you prefer a paper log, keep it in the vehicle and complete each entry before you start the engine for the return trip.
Record your odometer reading on January 1 (or the first day you use the vehicle for business) and December 31 each year.
Note personal trips as well as business trips -- a log showing only business miles with no personal miles recorded looks incomplete to an examiner.
Back up digital logs to cloud storage so a lost phone does not erase a year of records.

Does the IRS require a separate log for each vehicle?

Yes. If you use more than one vehicle for business, you need a separate mileage log for each one. The business-use percentage is calculated per vehicle, and mixing records from multiple vehicles into a single log makes it very difficult to substantiate the deduction for any of them.

What if my employer reimburses me for mileage -- do I still need a log?

If your employer reimburses you under an accountable plan and you submit mileage records to your employer as part of that process, those records serve as your documentation. You should keep copies. If the reimbursement is less than the full IRS standard mileage rate and you want to deduct the unreimbursed portion, you need a log supporting the total miles driven, not just the reimbursed portion. Note that employee business expenses are subject to additional limitations under current law.

Can I use my phone's location history as a mileage log?

Location history from a phone or mapping app can serve as corroborating evidence, but it does not automatically satisfy the adequate-record requirement on its own. Location data shows where you were and when, but it typically does not capture the business purpose of each trip. To use location history as part of your substantiation, you would need to annotate it with the business reason for each trip and pair it with other supporting documents such as client records or appointment calendars.

How long should I keep my mileage log?

The general rule is to keep records for at least three years from the date you file the return on which you claimed the deduction, which matches the standard IRS audit window. If you also use the vehicle for depreciation purposes, records supporting the vehicle's basis and depreciation history should be kept for three years after the year you dispose of the vehicle, since basis affects gain or loss on the sale.