|The Ford Thunderbird got away with a 3 speed.|
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.|
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!|