On the surface, the last claim, food miles, sounds plausible. It follows that imported food would cost more if the added cost of transport is factored in, but I recalled that local food often costs more than imported. I suspected that carrying tons of food for many families in one big eighteen-wheeler truck to a supermarket was probably as efficient as carrying pounds of food for a few families in a pickup to a farmer’s market, even though the semi got worse mileage and traveled farther.
Being a nerd, I did the math. First, I checked the internet for reasonable numbers to assign to the distances traveled, the mileages of the vehicles, and the amount of food carried per trip in the pickup versus the semi. Then I calculated how much gasoline it would take to move a pound of food in each vehicle – the truly relevant statistic rather than how far the food travels. I converted the answer to tablespoons, easier to visualize than decimal fractions of a gallon. No higher math required, just multiplication and division.
Let’s assume the pickup transports 500 pounds of food on each trip. It gets 15 mpg and travels 50 miles on each trip from farm to farmers’ market, so 50mi/15mpg = 3.33 gallons gasoline is used per trip in the pickup. If each trip moves 500 pounds of food, 3.33gal/500lbs = 0.00667 gallons to move each pound of food in the pickup, which is equivalent to 1.7 tablespoons of gasoline. [.00667 gallons x 256 T/gal = 1.7 T]
We can do a similar calculation for a big semi food truck. It gets only about 4 mpg, but it can carry 50,000 pounds of food. Let’s assume the truck travels 1500 miles, the oft quoted distance from farm to fork. So, 1500mi/4mpg = 375 gallons per trip. Divide 375gal by 50,000lbs and you get 0.0075 gallons of gas per pound of food. And, 0.0075 gallons = 1.9 tablespoons of gasoline, about 5/8 teaspoon of gasoline more than the pickup.
Guess what? The Sierra Club magazine agrees with me. The August 2008 “Ask Mr Green” column says, “A locavore's transportation footprint can actually be comparatively large, depending on loads and vehicles. Hauling 500 pounds of cabbage 50 miles in a small pickup, for instance, can burn about the same amount of fuel per pound of cargo as trucking 50,000 pounds 1,500 miles in an 18-wheeler. Plus, if the semi backhauls food, then it can be twice as efficient as a pickup that's returning empty or partially full.”
However, the family who drives 50 miles in their Prius (50 mpg) to meet their farmer uses more than five tablespoons of gasoline per pound of food if they carry back fifty pounds of food for the week. 50mi/50mpg = 1gal gas per trip. 1gal/50lbs = 0.02 gal x 256T/gal = 5.12 T.
If you can’t do the math, you can be fooled by false claims. The food miles fallacy is a perfect example of something that’s plausible but not true. When your teacher explained word problems, even though your eyes were glazing over, she had your best interests at heart.