According to the number of corners/hairpins and straights on the track you must choose proper gear ratios. For twisty tracks without long straights go for short ratios. This increases the vehicle acceleration but, on the other hand, decreases its top speed. For speedy stages with lots of straights and gentle bends you should choose long gearing which will allow you to reach high top speed, unfortunately at a cost of acceleration.
If the track consists of lots of hairpins but also has a couple of long straights (and usually you deal with this situation) the lower gear ratios can be set up with good acceleration in mind (for these hairpins) and then the higher gears can be stretched out to allow for higher top speeds (for those long straights). Remember only to test your settings on the shakedown first, to check if the gaps between gears are smooth, because it can often be a problem balancing the ratios when you're going for mixed low speed acceleration with a high top speed.