The value of a company car's personal use is taxable income.  That includes commuting, and the ability to use it on evenings and days off for personal use.  There is no set procedure in the tax law for you to add this income yourself to your tax return, it's supposed to be on your W-2.  If your employer refuses, there are one or two procedures that you could potentially use, although they would call the IRS' attention to your employer's failure to follow the law.  Best to get your employer to do it right the first time.