This view claims to accommodate the seemingly odd behavior of temperature in the last 15 years.

FORBES: Global Warming: Hotheads, Flatliners and Lukewarmers, Part One

But especially with hybrids such as the C-Max, he says, variables such as temperature, a driver's behavior and the engineering choices Ford makes to balance mileage and performance can make a big difference.

WSJ: Why Your Mileage May Not Measure Up