I drive around all day as part of my job too, but I don't seem to have this craving for something in the car with me. Perhaps a good start would be to get fast food that's at least a little better for you?

Personally, I'm completely addicted to the Swiss Oatmeal at Corner Bakery. It has some sugar in it, but the oatmeal fills me up for much of the day. For lunch type foods, it sounds cliche but I'll stop at Subway for a relatively low-calorie sandwich (I just can't stand the "Subway smell").