The Internal Revenue Services allows you to deduct certain transportation expenses related to your business. If you drive your own vehicle on a business trip, you may choose to take your deduction based on the mileage or to use the actual costs of operating the vehicle. Once you choose the standard mileage rate, you may not deduct depreciation or any other vehicle-related expenses. You must keep records of your business trips in a mileage log to support your deductions in the event of an IRS audit.
You must be able to provide documentation identifying the vehicle and proving that you are the registered owner. Your log should include verification of the vehicle's cost and the date on which you first used it for business purposes. This date determines whether you may choose the standard mileage rate deduction. You must elect the standard rate in the first year you use the vehicle for business or you will not be eligible for the deduction in subsequent years.
List the dates and destination cities for each portion of the trip. If only part of the trip was spent on business, note the dates on which you conducted business and the dates on which you were traveling for personal reasons. List the number of miles driven per day, splitting your mileage between business and personal use, if applicable. Keep any documentation such as gas receipts and receipts for tolls and parking fees. These expenses are deductible, even if you elect to use the standard rate for your mileage.
A raw mileage log is not sufficient support for tax deductions. You must also provide a written explanation of the business purpose for each trip. This written documentation is not necessary if other supporting documents can explain the circumstances of your visit. For example, hotel receipts and materials from a work-related conference may be sufficient to explain a trip to your destination city. You are not required to record confidential information such as client names and addresses in your mileage log, but you must keep the supporting data in a secure location in case you are audited.
You must keep your mileage records for as long as the IRS may need them to process your tax return. For most returns, this time period is three years after the filing date. If you filed an amended return to claim a credit or refund for a past year, you must keep your records for the later of three years or two years after paying the taxes due. If you amended your return due to a failure to report required income that exceeds 25 percent of the reported gross, keep your mileage log for six years. If you did not file a return for a particular year or filed a fraudulent return, you must keep your mileage records indefinitely.
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