Everything I know about this comes from Superbikes.... But the parts I'll tell you about most certainly apply to cars as well. :-)
There are two rules I always emphasize at the racetrack:
1) Three-Time World Champion Freddie Spencer always said "Go Fast in the Fast Stuff".
Everybody is slow in the slow stuff. If you want to beat somebody, it happens by learning how to go faster when and where it matters. Others have alluded to it here already: use the slow stuff to setup for the fast stuff.
Which brings you to rule number:
2) The quickest way to find the fastest way around a race-track is to learn it backwards.
Start at the fastest part of the racetrack, and ask "if I want to go fastest here, where do I have to be when I enter the straight? That means making that straight as LONG as possible (meaning you reach full-throttle earlier in the straight). So if I want to be able to be "here, this fast" at the beginning of the straight, then where and how fast do I need to enter the corner? .... etc ..... etc..... work your way backwards through the racetrack like that.
So the question is exactly as another commenter below has posed: which gear gets you the most speed at your braking point on the straight?
For me, in the car, I tend to think I can carry more speed through the corner and thus begin my straight from a faster start. But invariably the car understeers, scrubbing speed, and I can't get the car to turn so I end-up lifting the throttle and ruining my drive. Being a little more patient (going slower) earns me huge dividends on the drive out of the corner, and that equates to more speed all the way down the straight.
A Short-Shift (does this term work in the car-world?) can be very handy on occasion, when it lets you go WOT when you're tempted to get greedy, and it very slowly picks-up speed through the corner and is finally in the power by the time you're driving out. This is a very smooth way to exit a corner fast. But if you find you have to lift AT ALL, then you blew it and you should be in a shorter gear.
On the other hand, if you find you have lots of corner-exit left, then you aren't going fast enough and should probably be applying that throttle judiciously in a shorter gear.
Doe that all make sense?