Short answer
If you forgot to track your mileage, you can still rebuild the log after the fact, but only from evidence recorded at the time: GPS or location history, calendars, invoices, receipts. A log rewritten from memory alone is routinely rejected. The IRS gives the most weight to records made at or near the time of the trip.
Why is a log rebuilt from memory weak?
Vehicle expenses are among the expenses the tax code requires you to substantiate with adequate records or sufficient evidence corroborating your own statement. A general estimate does not meet that standard, and courts have repeatedly refused logs written long after the trips with nothing to back them up. The problem is not the date the log was typed; it is that nothing independent supports its content.
Can I still deduct my mileage this year?
Yes, if records made at the time show the trips: location history, calendar entries, invoices, fuel receipts. The step-by-step method is in our guide to reconstruct a mileage log; if the IRS is already asking, see mileage log for an IRS audit. To see what the miles are worth, use the mileage deduction calculator.
How does Google Maps Timeline fit in?
If location history was on, Google Maps recorded each drive when it happened: the start and end times, the places, and the distance. That is the part of a log people struggle to rebuild, and it was made at the time. What Timeline does not know is why you drove; the business purpose is yours to add, ideally from your invoices or calendar. You can export your Google Maps Timeline from your phone in a minute.
What if my Google Maps Timeline is empty?
Since Google moved Timeline to the phone, history is often off, or set to delete after 3 months. Before anything else, check how far back your export goes: the Google Maps Timeline mileage log shows it month by month, for free. For months with no record, only other records made at the time count.
To make next year easy, turn Timeline on now and set auto-delete to "Don't auto-delete activity" in the Timeline settings of Google Maps.
Common questions
Is it illegal to write a mileage log after the fact?
Writing a log later is not illegal; inventing trips or miles is. A log rebuilt from records made at the time, and labeled honestly as such, is a reconstruction. Its weight in an examination depends on the evidence behind it.
Can I estimate my business miles instead?
No. Car expenses fall under the strict substantiation rules of section 274(d) of the tax code, so an estimate without records is generally not accepted, however reasonable it sounds.
What if location history was off for part of the year?
Then the log has gaps. Fill them only from other records made at the time, such as calendar appointments, invoices or fuel receipts, and keep those records with the log. Do not fill gaps from memory alone.