QuickBooks makes it really easy to track mileage for your vehicles.
Once the company file is open, go to Company -> Entire Vehicle Mileage.
The vehicle menu allows you to choose or add a new vehicle. The trip start and end dates choose when the job is starting and finishing. Typically, this will be on the same day. The total miles are automatically calculated by the difference between the odometer start and end values. The customer job, item, and class associates the total miles with a customer, job, or class. The billable button allows you to decide if you want to bill the client for those miles.
A report can be created to show the total miles during a certain period by going to Reports -> Jobs, Time & Mileage -> Mileage by Vehicle Detail.
The current mileage rates selected in this picture are the 2013 standard mileage rates but you can bill your customers at a higher rate if you desire. Since the IRS and customers like to have a record of miles, QuickBooks makes it easy to keep track of this documentation.