I drive to work everyday. There are two routes I can choose. One is a normal 10-mile road that takes 40 minutes. The other one is motorway that is around 20 miles long, but only takes 20 minutess. I'm thinking that the normal petrol-mileages formula (as I remember) shows the petrol usage vs. the distance. Does it take the time into account? Suppose, in the extreme case, that the road is very congested, so the car is running but not moving; then the petrol usage could be very high. Back to my case, is there any formula to assess which route is more economic?