Vehicle Mileage Log for Taxes: 7 Mistakes That Sink the Deduction

Business mileage is one of the largest deductions available to people who drive their own car for work — contractors, agents, cleaners, consultants, anyone running a small operation out of a personal vehicle. It’s also one of the most commonly disallowed, and almost never because someone lied. It gets disallowed because the record is thin.

Here are the seven ways a mileage log goes wrong, and the column that fixes each one.

1. Reconstructing It in April

The single most common failure. Tax time arrives, the driver opens a calendar, counts appointments, multiplies by an average round-trip distance, and writes down a number. It might even be accurate. It is not a contemporaneous record, and it’s the version most likely to be challenged, because a reconstructed log has a distinctive smell: suspiciously round numbers, no odometer readings, and identical mileage for trips that couldn’t have been identical.

The fix: log at the trip, not at the year-end. Thirty seconds in the car before you drive off. A year of thirty-second entries is less total effort than one afternoon of reconstruction, and it’s the version that holds up.

2. No Odometer Readings

A row that says “42 miles” is an assertion. A row that says “start 88,412, end 88,454” is a record. The second one is internally consistent with every other row, ties to the year-end reading, and can’t accidentally overlap with another trip.

The fix: start and end odometer columns, with miles calculated by subtraction rather than typed. If you type the miles, nothing catches an error. If the sheet subtracts, a mistake shows up as an impossible number.

3. A Purpose That Isn’t a Purpose

“Client.” “Work.” “Business.” These are categories, not purposes, and a log full of them tells a reviewer nothing about whether any given trip was deductible.

The fix: name the who and the why. “Site visit — Henderson kitchen, measure-up”. “Supplier — Trade Depot, collect tile order”. “Client meeting — R. Alvarez, contract signing”. It takes four extra words and it’s the difference between a log that answers questions and one that raises them.

4. Counting the Commute

Travel between home and your regular workplace is commuting, and it’s generally treated as personal regardless of distance. Trips between work locations, out to clients, to suppliers, or from a qualifying home office to a job site are a different category entirely.

This is where a lot of honest logs drift, because “I was driving for work” feels like it should be enough. It isn’t, and the rules around home offices and temporary work locations have real nuance in them.

The fix: a “business / personal / commute” classification column on every trip, filled in at entry rather than sorted out later — and a conversation with a tax professional about your specific setup, because this is the area where the general rule and your situation are most likely to diverge.

5. Not Knowing Total Annual Miles

Business miles alone don’t establish a business-use percentage. You need total miles for the year too — and if you’re using the actual-expense method, that percentage is the entire basis of the claim.

The fix: record the odometer on 1 January and 31 December (or your fiscal equivalent). Two numbers, once a year. Everything else divides into them. If you keep a fuel log with an odometer column, you already have these for free.

6. Hard-Coding the Rate

Someone builds a beautiful spreadsheet, types the standard mileage rate into every formula, and then the IRS publishes a new rate for the following year — which it does routinely, and has occasionally done mid-year, splitting a single tax year into two different rates for two date ranges.

The fix: put the rate in one settings cell and have every row reference it. If the year is split, hold two rates with their effective date ranges and let the formula pick by trip date. Check the current published rate at the start of each tax year rather than trusting a figure that shipped inside a template — including this one.

7. Keeping the Log Somewhere Else

The mileage log lives in a phone app; the fuel log lives in a spreadsheet; the repairs live in a shoebox. At tax time none of them talk to each other, and the actual-expense comparison — which needs total vehicle costs and business-use percentage — becomes an afternoon of manual joining.

The fix: one file per vehicle, with the mileage log as a tab beside the fuel, maintenance, repair and insurance logs. The odometer column is the spine that connects them.

A Log That Passes: The Column Set

Nine columns. Four of them calculate themselves. The four you type take about thirty seconds, in the car, before you pull away.

The Vehicle Expense & Maintenance Tracker from ReadySheetGo includes exactly this as one of nine tabs — a business Mileage Log with a rate held in a settings cell so you can update it each tax year, sitting in the same workbook as the Fuel Log (which supplies your total annual miles from its odometer column), the Maintenance and Repair logs, and Insurance and Loan/Lease tabs. That’s what makes the standard-rate versus actual-expense comparison a lookup rather than a research project.

Both Methods Need the Log

Worth restating, because it’s the misconception that causes the most lost deductions: choosing actual expenses does not exempt you from tracking miles. The actual-expense method applies your business-use percentage to your total vehicle costs — and business-use percentage is business miles divided by total miles. No log, no percentage, no claim.

So the sequence is always the same. Track the miles. Track the costs. Compute both methods. Take the better one, subject to the rules about which method you’re permitted to use in a given year — which depend on what you’ve claimed before, and are worth confirming with a tax professional rather than a blog.

This is general information about record-keeping, not tax advice. Rules vary by situation and change over time — check the current IRS guidance and talk to a qualified tax professional about your circumstances.

For the full vehicle cost picture the mileage log plugs into, see the pillar guide: vehicle expense tracker spreadsheet. And for what those total costs come to over a year: how much does it cost to own a car per year.

Featured on ReadySheetGo

Vehicle Expense & Maintenance Tracker — Business mileage and vehicle costs in one workbook, which is what the actual-expense comparison requires. Nine tabs and 674+ auto-calculating formulas: a Mileage Log with a rate held in a settings cell, a Fuel Log with odometer-driven MPG and total annual miles, a Maintenance Log with next-due reminders, a Repair History with parts-versus-labour split, Insurance and Loan/Lease tabs, depreciation tracking, and a Dashboard cost-of-ownership summary for up to 4 vehicles. Works in Excel and Google Sheets, no macros. Instant digital download — $12.99.