You can setup an expense item called Mileage. Enter a default rate on the expense. Then when you use that expense item on a new slip it will show the rate. Just fill in the quantity with the number of miles.

Thanks for your response! Where is the default and how do I set up an expense item? Thanks again.

You might want to get some training from a certified consultant. The expense list is the icon that looks like a folder with a $ on it. Click on that and then click the plus sign to add a new expense.