Supposing your morning commute is about 10 miles by the shortest route available, but you can get to your destination faster by staying on the freeway, which goes about 2 miles out of the way. The extra 2 miles are all travelled on the freeways at optimum freeway speeds. The shortest route alternative requires that you get off the freeway at about the halfway point, and take a major thoroughfare which also moves very well at the time of day involved, but still involves the usual stops along the way.

So is it more fuel efficient to stay on the freeway and go a couple of extra miles, or to take the surface roads and minimize the actual distance?