Reimbursement for gas expense. How can I approach mom about this?

My mother who lives independently relies on me to transport her to get to her doctors appointments and other places she needs to get to. She will not drive herself nor use any form of senior transportation services. I live quite a distance so those trips cost me in gas expense. My mother has never offered to help cover the cost of the gas expense. I am not rich and it would help to be reimbursed. It takes one hour just to get to her house and an hour back. All I want is the cost of the gas I use to transport her to the places she needs to get to. How do I approach my mother about this without sounding like it is a burden on me. I would really prefer her to use other tansportation means and use our time together as a visit instead of being a chauffeur, I do not have the time to do both.