Fuels are blended according to the season (summer winter - welcome to Canada) and this is the biggest conrtibutor to fuel economy complaints.
I (presently) get about 6.7 to 7.4 L/100 pm on my old sh-box, a 1990 gm product 4 cyl auto.
In the winter it rises to 8 to 10 L/100km. The vehicles is always plugged in overnight. There are other factors that affect fuel economy in the winter (such as snow and slush) but they are not consistent factors (at least not for me).
The same goes for my minivan. Which incidentally has a trip computer and is always off by 1 to 2 L/100km. It is always too optimistic.
http://www.canadiandriver.com/articles/nm/mazdasiss.htm
Here is a link for Mazda's solution to idling problems. At least it promises to remove some of the wear and tear on starters. Maybe even save fuel.
QUESTION: Isn't this hard on the starter motor and wear and tear?
if you have an older model vehicle go to a junkyard and get another for dirt cheap. Canadian tire will also sell you a lifetime warranty starter as well so if you are planning to do this long term here are two solutions for you.
My 2nd hand starter was $50 and is now 1 1/2 years old.
Coasting depends on what your driving terrain is like. Other than shutting down at stop lights there are not many places in the city where I can shut down and coast. When I drive north of the city into the "hills" there are opportunities to shut down and coast down hill. Overall however it represents only 1 to 2% of my driving before I fill up. So not a significant savings for me. Besides constantly shutting down and starting becomes tedious after a while.
You could always install a vacuum gauge. It is a cheap and effective way of monitoring your engine and fuel economy.