Agree w/ what Delta Flyer said. Some additional thoughts... are you running climate control? In summer you might be running the A/C to cool off. In winter you might be running it with the defrost on to keep the windows clear. In winter, if it's cold enough, you might not be getting lean burn. Everywhere but the tropics or subtropics in winter you can also expect more drag from rain or snow. The opposite of that in the tropical zones where you would get the rain in the summer.