re: Company Vehicle : Do my employees buy gas or do I?

My employer reimburses me for all gas for my company car except when I am driving it on vacation. I pay with a credit card and send the receipt in with my weekly expense claim form. At the end of each quarter I have to submit a form that tells how many miles for that quarter were personal miles and how many were business miles. Based on the % of personal miles to the total and some valuation of the car, they say I received a benefit of x number of dollars. I am taxed on that amount. I think of it as something like a bonus where I am taxed on the amount of the bonus.