a car accellerates fastest when torque at the wheels is maximum. Horsepower is a somewhat confusing concept. If you know the torque curve of the engine and your gear ratios, you can always see at what point in the curve you will deliver the most torque to the wheels by comparing the output torques of the gears at the same vehicle speed. Say you have 1:1 4rth and a 1.25:1 3rd. Looking at it both ways, you have a 1.25 or a .80 ratio (.8 is the reciprocal of 1.25). In third, the engine will run 125% faster and multiply the engine torque available at that rpm 1.25x to the driveshaft, and would accelerate the car faster-but the engine torque is falling off quickly when you get up in revs. In this vehicle, if the 80% of the torque at 4800 rpm is greater than 100% of the torque at 6000, you should be in4rth (.8x6000=4800) In a VW it certainly is.
To keep it simple, I used 1.25(.8) for an example. In all cases, express the 2 ratios as a fraction and also as the reciprocal of that fraction to manipulate/compare output torques/rpm/gear/speed. The problem with using this info is that most of us don't have good torque curve info unless we go to a good dyno/operator.
If you do and analyze the info, you'll find that the old proscription is pretty close: unless your ratios are really shitty, you want to upshift at peak HP and your RPMs will drop to the area of peak torque. But if you have good data, you'll ussually find that despite your seat of the pants feelings, you're shifting way too late!
In any gear, the max acceleration is at the torque peak-which is at a pretty low rpm in most IT cars. phil