From a purely climatic point of view.
Assuming the following:
- For each calorie of food you consume, the equivalent of 9 additional calories worth of gas were burned to get the food to your plate (this is industrial food production).
- You eat 2000 calories per day.
- Your car gets 30 miles per gallon.
- Each gallon of gas contains the equivalent of 30,000 calories worth of energy.
So your daily food is about 2/3 of a gallon worth of gas, or ~20 miles of driving. If eating local and organic reduces the fuel per calorie of food by 50%, that’s the equivalent of reducing your annual driving by about 3650 miles, or about 25% of the average American’s driving. Certainly non-trivial, but not huge. If you figure in the additional equivalent fuel used to produce the gallon of gasoline in the first place, it’s more like 40,000-50,000 calories per gallon total, which reduce the miles-equivalent of your food to something more like 14, equivalent to more like 2500 miles annually.
Going car-free makes a much bigger difference.