My friend Ashwin (I don't know his team-bhp id) and I had brainstormed and googled up a lot on a discussion that started with how a 6th gear would help. Some of the conclusions we agreed on (please feel free to differ with explanation):
Before going ahead, this link will be very useful:
Puma Race Engines Technical Guide - How engine power and top speed are related Minimum number of gears:
This is an irrelevant question as of today as most cars have 5 forward gears as standard. But theoretically, it would be number required where each upshift would be done at the redline in lower gear to 1000 rpm in next gear till we get such a gear where we can no longer reach the peak power rpm. This number could be mostly 2 or 3 gears in most cars. For instance, most cars reach 40 Kmph in 1st gear. 2nd gear will be 40 Kmph at 1000 rpm. This 2nd gear will unlikely reach the redline before it hits the top speed.
Top Speed:
Top speed is roughly a function of max power, vehicle weight and other variable resistances (such as wind resistance, rolling resistance, etc. which can increase exponentially as speed gains). The link
Puma Race Engines Technical Guide - How engine power and top speed are related does have a rough table indicating maximum speed for a given power. Max power vs top speed graph is not a linear one as the resistance is not constant. Yet, in simplistic terms, for a given car, a more powerful (more peak power) will always result in higher top speed provided an appropriate gear ratio is available. More on this in the next paragraph.
Does my car need a 6th gear or more?
I'm quickly coming to the top of the thread. As mentioned above, for a given car, more power is supposed to pull the car to higher speed. It indirectly means that a car is supposed to attain its top speed in its peak power rpm. If a car reaches its top speed in the last/tallest gear in an rpm, which is higher than the peak power rpm, but at which the power is much lesser than the peak power, then another gear will help pushing the top speed further. For example, Linea with 1.3 MJD 86 ps (before BSIV) reaches 168 Kmph in 5th (last gear) at 4500 rpm. But if you look at the power curve of Linea 1.3 VGT MJD, power at 4500 rpm is just about 75 compared its near 90 ps at 4000 rpm (refer power curve:
http://www.petes.in/images/Fiat_Linea90BHP.pdf). If you look at the curve, the same engine produces 75 ps as early as 3000 rpm itself. So, another gear will run at an rpm anywhere above 3000 rpm at 168 Kmph will pull the Linea even to further speed. However, I'm not exactly saying that the 6th gear should be at 3000 rpm at 168 Kmph. If we do that, it may not reach 4000 rpm at all as the resultant torque on the front wheels are much lower than the resistance it faces on its way to 4000 rpm. Even then we may reach the best possible top speed, but will not be quicker ratio. From the table, 90 ps give a top speed of 110 mph, i.e. 177 Kmph. Lets assume this as the theoretical top speed of Linea. Lets assume that Linea's 6th gear reaches is at 3500 rpm at 168 Kmph (48 Kmph per 1000 rpm) and manages to go till 177 Kmph, it will be just about 3700 rpm. Compare this to a 6th gear ratio of about 44 Kmph per 1000 rpm, i.e. 3800 rpm at 168 Kmph. This later ratio will be much quicker to 177 than the earlier one.
Now you will know why we don't need another taller gear for Swift or Verna, because they already cannot reach the peak power rpm (4000 rpm) in their 5th gear itself. But i20 does 174 Kmph in its 5th gear at 4000 rpm. Since it has already hit (or very close to) the peak power rpm in 5th gear, it actually does not need a 6th gear. But the fact that it can touch the peak power rpm in 5th gear means that it can benefit from another taller gear in fuel efficiency (later about fuel efficiency).
Best gear ratio for best fuel efficiency:
As we know, an engine burns some minimum fuel per a given amount of time irrespective of the speed (either idle or slow or fast) unless it is a hybrid. So, it is by simple rule better to be fast. But as speed gains resistance increases and more energy is required to pull the car. So, the engine ends up burning more fuel. Different manufacturers and research indicated that 60 - 80 Kmph is an optimum speed for the internal combustion engines. I have no links for this number (please provide if you have one). Car manufacturers also keep minimum fuel burning at around 1100 rpm (i.e. when a gear is engaged, ECU trying to maintain 1100 rpm). So, ideally the most fuel efficient gear should be a gear that runs the engine as 1100 when the car speed is 60 Kmph, i.e. about 55 Kmph per 1000 rpm. But then to run at 60 Kmph (about 40 mph), an minimum power of 8 ps is required. 1.3 MJD engine just manages to have about 8 ps at 1100 rpm (refer the graph again
http://www.petes.in/images/Fiat_Linea90BHP.pdf). So, theoretically we could have a gear which run at 60 Kmph at 1100 rpm. This should be the most fuel efficient speed and gear combination. But if the engine has less than 8 ps at 1100 rpm, it has to have a gear that produces just 8 ps at 60 Kmph (calculate whatever rpm it is). That should be the most fuel efficient gear.
But I cannot drive at 60 Kmph all the time just for efficiency
So, what speed you want to drive at? 100 Kmph? Ok, lets choose a gear ration that is just suitable for this. Minimum required power to drive at 100 Kmph (60 mph) is about 20 ps. In the graph again, 1.3 MJD produces 20 ps as early at 1350 rpm. So, we need a gear that runs at 1350 rpm at 100 Kmph (i.e. about 74 Kmph per 1000 rpm, am I crazy??). As far as I can say, that is the gear you need if you want to drive at 100 Kmph. But as you know, it will be really difficult to accelerate from such gears. Well, I could always downshift if I'm in a hurry... just give me such a gear to cruise at 100!!
By now, we can see that we can benefit a lot on fuel efficiency from taller gears and even on top speed in case of some of the existing cars. I think Toyota Altis did the right thing for a heavy car with 90 ps. More number of gears also benefit in city driving conditions where we want to find most fuel efficient gear with right amount of pull at various lower speeds. More than 5 gears manual shift may bog down drivers and thus we may end up not getting the benefit, but in an automatic, higher gears are a must according to me, unless it is a continuously variable (infinite) gear system.