The EPA numbers are real but not realistic.

In the 1970s, Congress mandated the use of EPA numbers or worse because
car sellers were providing dubious numbers taken under optimum
conditions with test drivers who knew how to squeeze every mile from a
gallon of gas. The EPA was employed because it was already in the
business of certifying that cars met emissions standards, and because
the EPA measured what came out of the exhaust, it could calculate the
amount of fuel that went in.  But the original EPA numbers were
unusually pessimistic, and almost every driver got better fuel mileage,
so either a new test was introduced or the numbers were arbitrarily
adjusted.  The adjustments have changed a few times over the years, but
I believe they tend to be roughly 1/3 too high for the city mileage and
1/3 too low for highway mileage.  Alsohybrid vehicles receive a
different test whose fuel economy results can't be compared to those
those of conventional vehicles, and hybrids often show higher city than
highway economy because during a large portion of the test only the
electric motor is run.

Many people also criticize the tests for being run on a dynamometer
rather than on the road, but this probably isn't much of a factor,
except maybe for trucks and highway driving.

«» wrote:

It doesn't matter whether you agree or disagree; the facts don't change
to match our opinions.

Why would Mobil say its synthetic does not improve gas mileage "one
bit," especially when it sells for 2-3 times the price of its
conventional oil and therefore gives Mobil a strong incentive to have
customers buy synthetic (they advertise Mobil 1 much more than their
conventional oils)?

The main causes of improved fuel economy are engine break-in, traffic
conditions, and driving habits.

Before the days when computers regulated engine idle speed, it was
common for the idle of a new engine to increase after the first
5,000-10,000 miles of driving because the engine had broken in and
loosened,  and I remember my carbureted Corolla needing its base idle
decreased by 200 RPM when it was a year old.    The Toyota manual said
the idle didn't have to be adjusted after 30,000 miles, indicating
engine friction didn't improve significantly after this..
Computer-controlled cars automatically make the adjustments, and some
are also programmed to make arbitrary changes at 30,000-60,000 miles.

Traffic conditions have been shown to make fuel economy commonly differ
by over 2:1, and driving habits can make a large difference in city
fuel economy.  Measurements taken by my employer showed city mileage
differed by 20% for the same vehicles driven over the same city routes
at the same times of day.  I strongly suspect your 2 MPG improvement
was caused by changing traffic conditions and, more importantly,
driving habits, but to find out for sure, have somebody put
conventional oil in your engine without your knowledge.   The test
won't be valid if you know of any change because it's been found that
drivers unconsciously change habits significantly when gas-saving
chemicals or gadgets are installed, whether they're completely useless
or not.

Frankly if you achieved a 2 MPG improvement solely due to using
synthetic oil, GM would be putting synthetic in all their vehicles so
they could sell more gas guzzlers and still meet CAFE standards.  But
as matters stand, they specify synthetic for only a few
high-performance engines, and with at least one of them it saves GM
some money by allowing the elimination of an oil cooler.

While most of the tricks I see in the media on boosting your milage with
driving tricks, there is one that I see so often that is not true.

This "trick" says to be VERY easy on throttle, barely cracking it.  This
is not true, especially with stick shift.

If it were not for "power enrichening,"  spark-ignition engines would be
most efficient at full throttle.  But because of such enrichening, max
specific fuel efficiency comes at around two-thirds to three-quarters
throttle.

With stick shift this means a good amount of throttle, say half or so,
but short shifting, keeping rpm down.

Unfortunately, throttle position with an automatic can change shift
points, causing upshifts to be at higher rpm.

However, even so, best fuel efficiency even in automatic still requires
far more than merely cracking throttle above idle.  Probably with most
cars it would be about a third to half open or so.