Check the sidewall of your tires. Read the small print on the sidewall
and you'll find a maximum load rating and related tire pressure.
It is not recommended to exceed the tire pressure listed for safety
reasons. However, increasing pressure up to that limit can result
in increase handling performance.
A practical method of setting ideal tire pressure
is to make chalk marks on the sidewalls, at 90 degree intervals,
running from the wheel outward to the beginning of the tread. Drive
the car in the manner you intend to use it, then inspect the sidewalls
of all four tires. Contact with the ground will have rubbed off some
chalk. Adjust pressure until the chalk line remains to the point
where the sidewall and the tread edge meet. If pressure is too low,
you'll rub off chalk on the sidewall. Do not be surprised if you
wind up with a pressure setting as much as 10 psi over stock. This
should be repeated after each step of suspension modification, as
new pieces will change the pressure requirement for optimum tire
There is more to the story than keeping all four tires on the
ground; you want them to maintain the proper angle to the pavement
so the contact patch of rubber is as large as possible. That
is what proper wheel alignment does.
Simply changing the alignment settings can make
a dramatic improvement in grip. What to change, and what the change
will produce, requires a brief explanation of some terms with which
you may well be familiar. A car's basic handling characteristics,
when pushed hard, fall into one of three types: understeer, oversteer or neutral.
Understeer is the condition when
the front end has less grip than the rear. If pushed beyond its limits
of adhesion, the car's front will slide before the rear tires lose
grip. In oval track racing, it's known as 'push.'
Oversteer is the condition when the rear end has less grip than the
front. Pushed to the limit, a car with oversteer will have its rear
tires break traction in cornering while the front stays stuck.
Neutral, of course,
is when the whole car slides at the limit. If you achieve this, have
the car bronzed; it doesn't occur often the the real world. Ideally,
you want a little understeer, because it is both more predictable
(you can feel it coming) and because, when all else fails, you're
usually better off leaving the pavement front end first. You have
more time to close your eyes and duck!
Remember, finally, that we are talking about handling
the limit. Handling below the limit is more a function
of driving technique than suspension geometry or spring rate.
*quoted from TRD USA Book - Toyota Cars: Built to Perform