Driving miles plus cycling miles per gallons of fuel. Include drinks if you buy them instead of tapwater.
I did not keep track of my driving miles or gasoline, since I was more interested in cycling miles. My truck is old and gets poor mileage, but I drive it less than weekly. In driving+cycling mpg I may still get less than some drivers who don't have bikes. People who have both bicycles and motorcycles probably have the highest numbers.