Your best bet is to look at a dyno chart of your car and see where all the power is made.

Here's a quick '101' scenario...

Let's say peak power is at 5000rpm.
Let's say the power curve is symmetrical about the 5000rpm line.
Therfore, you car is making the same power at 4500rpm as it is at 5500rpm (which is less than at 5000rpm).
Let's say when you change up a gear the rpm drops 1000 every gear.
So if you're in 5th doing 4500rpm and you drop to 4th and maintain your speed you're now doing 5500rpm, (and vice versa).
So, to keep in the max power band you should bechanging gears at either 4500 or 5500 depending on whether shifting up of down.