I have no idea about cars so the question might be a bit silly but I can't manage to find the answer anywhere on the web.
Basically, as far as I have undestood through my own experience; (Please ignore the numbers, it's just an example) I can be using 4th gear to go at 80 km/h while reving at 2000 rpm and using 6.0 litres every 100 km but on the other hand I could be using 5th gear and I could be going at the same speed, 80 km/h while reving much less at 1400 rpm and using less petrol, something like 5.5 litres every 100 km.
This afirmation must be so wrong because it makes no sense to me that there aren't many cars with more than 6 gears. It all depends on the torque? Does it depend on the horsepower? I have no clue.
I have a 1.7 Opel Corsa from 1997 which using 5th gear (max I have) while reving at 3000rpm I manage to go arround 120km/h, but of course with 4th reving at 3500rpm I manage to go nearly at the same speed.
So, why aren't there more gears?