Did you know that the IRS has given examples of acceptable partial year mileage logs? (I didn't until recently.)
Under IRS Reg. Section 1.274-5T(c)(3)(ii)(C) the IRS has actually said as an example that maintaining an auto log for three consecutive months showing a percentage of business auto use plus showing via paid bills and other receipts that your business continued at the same rate for the rest of the year will provide sufficient corroborative evidence if other circumstances don't change (getting a second car for the business, for example.)
The IRS also gives another example of keeping a log for the first week of every month for the whole year as also providing corroborative evidence, but what is the point of doing this - you might as well keep a log for the whole year!
Here are the basics of how the IRS describes the three-month test:
*The taxpayer uses her vehicle for business use.
*She and other members of her family use the vehicle for personal use.
*The taxpayer keeps a mileage log for the first three months of the taxable year, and that log shows that she uses the vehicle 75 percent of the time for business.
*Invoices and paid bills show that her vehicle use is about the same throughout the year.
According to this IRS regulation, this three-month sample is adequate to prove 75 percent business use.
Click on the link below to look at the actual regulation as the IRS states it (thanks to the Bradford Tax Institute for the Section and info on this easier way to substantiate auto mileage.)