There is no one perfect gear for all road courses.
 That's why professional race teams carry multiple
gear sets to the track.
   If you're only racing at one track, it's a lot
easier.  What is the top speed you'll expect to attain?
 Try and select the gear that places you just past the
peak of the torque curve at that speed in 4th (1:1) gear.
    That will be a good place to start.  But expect to
make a change or two as the season progresses.
    Optimally, you'd want a 8 inch rear, with multiple
center sections.  The center section can be swapped out
in less than an hour the first time.  All it takes is a
couple of hand tools.   Even a non-mechanic can do it.
    The 8.8 is plenty strong enough, but since it has
an integral carrier changing gears is time consuming
and requires special skills and tools.  A 9" rear is
stronger, but also requires more power to run it.  A
well built 8" will last forever on a road course, will
be lighter, and will deliver a little more HP to the
ground.

Kinda following up on what boB says..... (and I hope you mean a race track
when you say "road racing")..... there is no finite answer toyour question.
30 years ago, we would have said the best gear ratio is the one that has you
at max RPM in high gear (direct drive at that time) just before you have to
apply brake at the end of the longest straight. With the equipment that is
available now, and the money willing to be spent, the equation has grown to
many, many more steps than what we have been use to.

The ideal situation has us being able to keep our RPM in the area where our
peak torque is being developed in acceleration conditions and where peak HP
is being developed during sustained high speed conditions. The ideal
situation also has us in "direct"... that gear that gives us a 1:1 ratio to
avoid gea losses.

The question you have offered is difficult to answer.... add the query
regarding 'economy' and all bets are off.

Decide on your priorities and then ask te question again stating what you
expect *within reason*. .....

The short answer I think is that you'll want more gear once you get
out onto the track.  In my 00' GT the 3.27:1's are ok but in lower
gears take more time to come back up to torque peak.  I have 3.42's in
another car with much less power and coming out of the corners I get a
better pick up than with the Mustang.  Both have the BW T-5
transmission.  

                            ...Ron
--
68' Camaro RS
88' Firebird Formula
00' Mustang GT Vert