mileage expenses

It takes a little to set up what you need in the event of an audit, but ...

Use a spreadsheet
One cell has to be reserved for the starting odometer reading on 1 Jan xx, and another cell for the ending odometer reading on 12/31/xx

Each row should have a cell for the date of the start of the trip
the odometer reading at the start
where you are leaving from, and going to
the odometer reading when you arrive
the date of arrival.

I set up a cell to do the math for the number of miles traveled per trip, and then I summed that column.

Use the business CC for all fuel and oil you buy, both business and personal.

End of the year you do the math to determine the total number of miles driven all year (ending odometer less starting odometer), this is both business and personal miles.

Do the math to determine the percent of business use and the percent of personal use

Then do a journal entry debit owner equity drawing and credit the fuel/oil expense account for the percentage of personal use.  You have to do the math first though, multiply the figure for the percent of personal use times the total for the expense account and use that number in the journal entry.

At that point you are done, other than using the total business miles as the input for the IRS standard deduction.

Business books do not have to match the IRS filing, in fact if you sell inventory items it will not, since QB uses FIFO (QBO) or average cost (desktop) and the IRS does not.

@Danbrich 

It's important to point out that you cannot take depreciation on your car if you use the standard mileage rate but you can if you use the actual expense method.  Depending on the value of your car, that can be a considerable tax deduction.