This article is from the VW Performance FAQ, by with numerous contributions by Jan Vandenbrande others.
A:Yourself. Most people only utilizes a small portion of
their car's capabilities, and often do not know how the
car handles under emergency conditions. Almost EVERYONE
can benefit by taking a performance "Driving School" from
one of the local clubs (e.g. SCCA, ~1/2 day, inexpensive,
fun) or from a performance driving school (e.g. Skip
Barber, Bob Bondurant, etc, expensive, fun). It is
probably the biggest single improvement you can make and
it's a skill you take with you no matter what car you are
driving.. In every day driving it may make the difference
between an accident and avoiding one!
The next question you need to ask yourself is *why* you
want to improve your car's performance. Do you want to
impress your friends? Do you want to blow away other cars
on the street? Do you want to compete, and if so, what
type of competition? There are all kinds of car
competitions: Autocross, road racing, rally, concours,
drag race, and so on?
Will you be using this car for your daily commute or will
it be purely used for competitions? Depending on what
you want to do, you may want to follow a very different
path to enhance performance.
What runs well on a track may not be acceptable or barely
drivable for a street car (clearance, noise, hard ride,
rough idle, bent rims, the law...). Additionally, if you
want to race in a club, cars are categorized depending on
their power and handling, and to what extent they have
been modified.
For example, it may be better to leave your car stock
than to make certain modifications. Most classing
structures allow only certain modifications, and if you
do somethign else, you'll be bumped to the next category.
For instance, in SCCA Solo II autocrossing, Stock-
category cars must run on rims that are the same size as
the originals. If you go with a wider rim, you will have
to run in the Street Prepared category. There, you would
also have to lower and stiffen your car and replace your
entire intake system in order to be competitive.
 
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