Does Fuel Efficiency Improve over time and mileage covered?

[architect]

While there are threads on fuel efficiency (and quite active ones!), I want to know if the Fuel Efficiency actually improves over time for most cars, both
from the technical point of view and from owners' experience.
Is there an increase in FE over time for cars? By what reading on the odo does it stabilise? Logic says it will improve as running in gets over, but what
about some people who report an improvement in FE upto 20,000 kms or so? Is there a time (or mileage) from which one can expect the FE to actually start
reducing? What about some European Cars which are supposed to run for 300,000 miles (approx 480,000 kms) without overhauling? Would they have a constant FE
from the end of Running-in till a mileage of 500,000 kms?
The answer to this question could also help a lot of people in deciding which cars to buy, especially those who intend to drive it beyond 100,000 kms, as it
would help them in the number crunching if they know the FE will stay constant till a certain mileage.
Thanks in advance!

[rranjith_kum]

I have thought of this so many times too.
but as far as i have experienced fuel efficiency reduces very quickly over the period of mileage for a vehicle with unrefined engines.
my scooty pep is one among them just clocked about 31k started to show its aging (don't know whether its got to do with its size but its was never abused)
i remember reading in somewhere honda vtec engines are delight after about 50k kms.

reg european cars its matter of the quality and refinement during the building process that determines the fuel efficiency over the life i guess.