A mileage log is a record of business trips that satisfies the IRS substantiation requirement under § 274(d). Without a defensible log, the deduction can be disallowed in an audit — even if your actual miles were legitimate.
Required elements per trip:
- Date
- Business purpose (which gig, which client)
- Starting and ending locations OR start and end odometer readings
- Total miles
Acceptable formats:
- Handwritten notebook
- Spreadsheet maintained throughout the year
- GPS-tracking app (NeighCheck, MileIQ, Stride, etc.) that auto-records each trip
- Rideshare/delivery app trip history exported as a CSV
"Contemporaneous" means recorded near the time of the trip, not reconstructed at tax time from memory. Reconstructed logs are a top audit trigger and frequently disallowed. The IRS uses calendar entries, email records, and phone GPS data to verify logs in audits.
Common mistake: relying only on the totals shown in the rideshare/delivery app. Those typically only include "active" miles (with a passenger or delivery). Miles between gigs, miles to/from the gas station during a shift, and miles in service of the business but unmatched to a specific delivery are all deductible — but only if you logged them yourself.