I was asking the same sort of question recently.
http://forums.pelicanparts.com/porsche-911-technical-forum/833746-rpm-shift-points-economy-cars-vs-racing-cars.html
This might be worth adding here:
First, look at the RPM vs HP/Torque chart on the left.
The engine puts out peak HP around 5300 rpm (regardless of gear) and peak torque is at 4000 (lower curve)
So, you’d want to be in that 4000-5300 range ...for optimal engine performance.
Next, see the gearing chart on the right
Note that for a given speed (x-axis), you can be in any of the 5 gears (with varying RPMs, in theory)
Let’s say you shift from 1st to 2nd at 3000 rpm (top orange circle).
At that moment, you’re now in 2nd gear, and going 20mph.
This puts you at ~ 2000 RPM (y-axis) in 2nd gear. (lower orange circle...)
As per graph #1, at 2000 rpm, the engine produces very low HP/torque.
In contrast, let’s say you you shift from 1st to 2nd at 6000 rpm (top green circle).
At that moment, you’re now in 2nd gear, and going 42mph.
This puts you at ~ 4000 rpm (y-axis) in 2nd gear. (lower green circle...)
At 4000 rpm, the engine is producing lots of HP/torque.
With the higher shift, you are maximizing time spent at the higher part of the power curve.