Vehicle expenses are one of the largest deductions available to an owner who drives for work, and one of the most commonly lost. Not because the rules are obscure, but because the deduction depends on a record kept while the driving happens, and almost nobody keeps one until the year they wish they had.
There are two ways to claim the cost, only one of which is usually available after the fact, and a set of trips that do not qualify no matter how the year went. Here is how the deduction works and how to set the books up so it survives a question.
Standard mileage or actual expenses: pick once, deliberately
The standard mileage method multiplies your business miles by a per-mile rate the IRS publishes each year. That single rate is meant to cover fuel, maintenance, repairs, tires, insurance, registration, and depreciation, which means you cannot deduct those separately on top of it. Parking and tolls incurred on a business trip are deductible either way, and so is the business-use share of interest on a vehicle loan.
The actual expense method does the opposite: you total everything the vehicle genuinely cost for the year and deduct the business-use percentage of it. That percentage comes from your miles — business miles divided by total miles driven — so this method still requires a mileage log. It tends to win for expensive vehicles, heavy repair years, and vehicles used almost entirely for business; the standard rate tends to win for an ordinary car with high mileage and low running costs.
The order matters more than most owners realise. If you want the option to choose between the two methods over the life of a vehicle you own, you generally have to use the standard mileage rate in the first year you place it in service — claim actual expenses that first year and the standard rate is off the table for that vehicle afterwards. A leased vehicle is stricter still: whichever method you start with applies for the entire lease. This is worth one deliberate conversation with your tax preparer in the year you buy, not a default you fall into.
What counts as a business mile, and what does not
Commuting is the trap. Driving from home to a regular place of work is personal, however early you leave and however much you are thinking about the business on the way. Driving between work locations during the day is business. So is a trip to a client, a supplier, the bank, the post office, or a job site, and so is the drive home from a temporary work location outside your normal area.
The exception that changes the arithmetic for a lot of owners is the home office. If your home is genuinely the principal place of business — you administer the business there and you do not have another fixed office — then trips from home to a job site or a client are business miles rather than commuting, which can convert the single largest block of personal driving in the year into a deduction. It also has its own requirements, including exclusive and regular business use of the space, so confirm it rather than assume it.
Personal errands folded into a business trip do not poison the whole trip, but they are not deductible either. The practical standard is that each trip has a business purpose you could state out loud, and the log says what it was.
The log is the deduction
A mileage deduction is only as good as its substantiation, and the requirement is specific: date, miles driven, destination, and business purpose for each trip, recorded at or near the time of the trip. A number reconstructed in April from memory and a calendar is exactly the kind of estimate that gets reduced or thrown out entirely, and the amounts involved are rarely small.
Any method that produces those four fields is acceptable. A mileage app running in the background is the least painful, because it records the trips as you drive and leaves you only the business purpose to tag. A notebook in the glovebox works perfectly well if you actually write in it. What does not work is a bank feed, because the feed knows you bought fuel and nothing at all about where you drove.
Record your odometer at the start and end of the year as well. Total miles are what turn business miles into a percentage, and without them the actual expense method has no basis to compute and the standard method has nothing to corroborate it.
Getting it into the books without contaminating them
For a vehicle owned by the business, the costs sit in the books directly and the personal-use portion is backed out at year end from your log. For a vehicle you own personally — which is most owners — the cleanest treatment is to leave the running costs out of the business entirely and have the business reimburse you for business miles at the standard rate under an accountable plan. The reimbursement is a deductible expense to the business and is not taxable income to you, and it keeps every personal fuel and insurance charge out of the accounts, which is where they cause damage.
The failure mode to avoid is the middle ground: running some personal vehicle costs through the business card, deducting mileage as well, and double-counting the same expense in two forms. Pick one treatment per vehicle and hold it for the year.
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