I have two possible routes to work. One takes about 25/30 mins, is around 28 miles and is abou 60% motorway/60 mph roads. The other takes about 25 to 40 mins, is around 23 miles, is only about 30% motorway and the rest is 30/30 mph roads. Which one would be best, economy-wise for fuel?
I would go for the 23 miles. The 28 mile route should give a slightly higher mpg, but not 22% higher which is what you would need to make the first route cheaper.
Fill your tank at the start of the week and then just go the longer route. Fill up again at the start of the next week and go the shorter way - which ever way took less fuel is obviosly the cheapest.