Fuel is not the biggest expense in owning a car, just the most prominent. Also the lighter a car the more life threatening the injuries can be in a crash. At some point higher mpg results in higher total cost.
It is for me. I get about 24 mpg from my Nissan pickup. Insurance runs me about $350 a year. Repairs are perhaps $250/year. Gas however costs me about $800/year. More than everything else combined.