If it’s December, it means it’s time for the IRS to announce the 2011 optional standard mileage rates. These rates are typically used by businesses to help calculate mileage expenses for employees. And just when you’ve started to remember the optional rate issued by the IRS, the agency changes it.

And so it has for 2011. Effective January 1, 2011, the standard mileage rate for use of a car for business miles driven will be adjusted upward to 51 cents per mile.

According to the IRS:

The standard mileage rate for business is based on an annual study of the fixed and variable costs of operating an automobile. The rate for medical and moving purposes is based on the variable costs as determined by the same study. Independent contractor Runzheimer International conducted the study.

As a reminder, companies with personnel policies about their mileage reimbursement should consider updating their policies immediately to reflect this change. In the future, employers can draft a policy that states that their standard mileage rate will be consistent with the IRS’s rate without reference to a particular number.