How many gears is enough?

The Ford Thunderbird got away with a 3 speed.
These days when you're looking for a car one of the biggest things they advertise is the number of gears it has. Commonly todays cars have 5 or 6 with 6 quickly becoming the norm.

 First I should explain the importance of a transmission. An internal combustion engine produces a certain amount of power depending on the speed it's turning. Lets take an average car as an example. The Honda Civic produces 128 Ib/ft of torque. Perhaps you think thats quite a lot. However if you've ever tightened bolts using a torque wrench you know that the human arm can easily produce 128 Ib/ft. So perhaps your engine isn't as power as you thought.

Therefore the transmission steps in to provide the power to move the car. When your car is in first gear you'll notice it's able to get moving quickly but won't go very fast. First gear works to multiply the limited torque produced by the engine, but at the cost of top speed. This is where the need of more than gear comes into place, finding the balance between power and top speed.

However years ago cars lacked all the technology we have today and they managed with 2 or 3 gears. Another difference with older cars is torque is only made at one point in the rev band. For example a car might make 185 ft/lbs @ 4800 RPM. These days engines provide torque over a broder curve than anything in the past. Turbo charged engines generally make peak torque starting just off idle and all the way up to 5000 RPM. Essentially this means there is more usable torque therefore you should need less gears to keep the car in it's range because it's much larger.

The Lexus IS-F has an 8 speed transmission. 
With this information in mind cars today should require less gears than before due to more efficient engine technologies. However the very opposite has happend. Today transmissions are equipped with 7 or 8 gears and 6 has become the norm. So I ask when does it stop? What is the ideal number of gears for a car? At this point it seems manufactures are just competing with each other by making a car with more gears. This in turn adds additional cost to the car.

This trend however has only seemed to affect ICE (internal combustion engine) powered vehicles. Electric cars are running with single speed transmissions and can accelerate very quick with a reasonable top speed. The advantage to a single speed transmissions is no time lost changing gear and no fuel wasted in between gears. This of course is done because electric cars make maximum torque from 1 RPM to redline or very close.
The Tesla roadster uses one gear!
I know what you're thinking though. "If gas cars had only one speed it would be revving far too high on the free way and wouldn't be quick enough in the city." So maybe 1 speed isn't the answer, but how much gas does a 7 or 8 speed transmissions really save you over 5 or 6 gears. Perhaps we're concentrating too much on transmissions and should focus on other fuel saving measures, weight anyone? The amount of fuel saved or perhaps slight performance increase may not even be warranted for the amount of money it takes to design and manufacture. I think auto companies could spend all this money on something else. I mean pretty soon we're going to see cars with more gears than a transport truck.