Why having a mileage log (or not) could determine the direction of an IRS audit.
The business record you probably hate to maintain the most is the one record that may receive special attention in an IRS Audit:
The mileage log on your vehicle.
Once your IRS audit begins, early in the process, the IRS examiner will probably ask to see this record. If the record is missing or lacking in substance, the Examiner may conclude that your other business records are lacking as well. The IRS believes that a taxpayer’s failure to keep a proper and contemporaneous mileage log on vehicles indicates that the business may not be operating in a business-like manner.
If you want to avoid more trouble than necessary at an IRS audit, keep a good mileage log. It shows you know the rules and respect them. Audits can end early because the taxpayer presents a good mileage log. It’s indicative of a business that keeps complete and accurate records. If not, you could expect a much longer and more invasive audit.
Here is a good article about how to keep a mileage log. There are links to commercially available resources for your log.