Of course, the first question must be whether every combustion engine needs a transmission and if so, what the reasons are. We answer the first question with an imaginary experiment. We take an internal combustion engine that is not very powerful and instead of a manual transmission, we put a simple gear drive behind it.

A kind of lawn tractor with a clearly limited maximum speed would make sense in general. This means that it does not use the power or torque of the engine to achieve the highest possible top speed, but also to be able to drive on possibly very steep meadow land.

Of course, such a vehicle needs an engaging clutch, because driving off together with starting the engine is not allowed. And yet it could be that you won't be completely happy when driving with this design. Because even a small combustion engine distributes its torque differently depending on the speed. So if you drive up a short gradient in the garden and want to do it slowly, there can be problems.

The engine bucks because it has too little torque with so little throttle for the uphill. It would be perfectly sufficient to go much faster uphill, but there would be the shock too hard for man and machine. It is similar with (real) off-road vehicles. There is a so-called off-road gear that feels a bit strange. With a lot of engine speed and sometimes there is some noise, one suspects a certain advanced speed, but the car just crawls up the steep mountain.

It is even more interesting when all four wheels have dug in. A normal first gear is often useless there. Then there is a lot of gas and if you engage the clutch, the engine speed drops and from the great 180 Nm at 3000/min in the picture above only 30 or even less remain. From this we learn that there must always be a lowest gear that rescues you from tricky situations.

Example in normal road traffic: Your vehicle is registered for a caravan, which is just under the legal limit of what your family wants to take with them. On the motorway this works well and country roads are no problem. They even try a steeper pass road and the vehicle can manage that too. Then comes a similar pass, where they have to stop in the middle of the steepest gradient because of the people in front of them.

It is best to start counting the money in the holiday fund, because this can be expensive. Hardly anyone has enough experience to start up here again without ruining the clutch. You tend not to want to overload it and let it come slowly even in the pressure point. This works well until it starts to stink even in the car. At the latest now you can plan a somewhat larger amount for a new clutch.

You would have had a tiny chance if you had let the clutch come quickly within reasonable limits and should proceed to avoid touching it again. You could also have waited for the traffic jam on a relatively flat stretch of road or, probably unsolvable in reality, rolled back there. But now you know why there are gradients indicated for the permissible trailer load, which you can easily manage when driving continuously.

What are the test criteria at the manufacturer and without trailer? Very simple: It must be possible to start up on a certain gradient with four people and luggage without any problems. That would then be the suitable first gear. In practice, this is only needed for about one car length. It only makes sense when starting at traffic lights to protect the clutch. Afterwards, shift as quickly as possible to the highest gear possible for the traffic situation. And the highest gear, how do you find it? That's where things get trickier. With only four gears, it was easy. That's where the car should be at its catalog value. So if the highest power was reached at 6000 rpm, the one on a flat road should also be reached in fourth gear. It was then the maximum speed. That's all. Of course it was clear that you don't drive for hours because it would have affected the life of the engine. But there was no need to worry, the traffic conditions were still far from being comparable to today's.

Whenever you deviate from this gear ratio to v_max, this will have a negative effect on the top speed. If you over-rev the engine slightly, this is good for accelerating to that point. If the motor does not reach its rated speed (speed of highest power), this is called a soft gear. However, it is possible to reach even higher speeds than the previous maximum speed on longer downhill runs.

And so there were two designs with the first five-speed gearboxes at the latest: For good acceleration and sporty driving, the fourth gear, which was formerly designed for v_max, was now changed to the fifth and the new gears 2 to 4 were now more closely graded between 1 and 5 (picture above). Or for the fuel savers, engine protectors and noise sensitive ones: The four gears were retained and the fifth one was in principle placed over it as a gentle gear (picture below).

When it comes to the question of how the gears in between are laid out, we pick out the diagram with the large gear jumps, which is more difficult in this respect, and present it again below. Here you can see that although the distances between the first four gears are relatively even in terms of angle, the jumps in engine speed differ considerably. By the way, the last gear moves closer and closer to the second last.

But the speed jumps go from 2700/min between 1st and 2nd gear over 2200/min, 1700/min down to 1200/min. The question arises what would actually be the ideal speed jump. But this is only relevant for sporty driving. Fuel savers often even skip a gear on the flat, e.g. 4th in the diagram above, because shifting gears costs more fuel than the intermediate gear contributes.

This engine has e.g. its highest power at 5400/min. In order to run it all times within its highest possible power-range, a transmission with at least at upper gears 1800/min speed difference is required. Then one would bring it up to the final speed, only then change gear and would never fall below 80 kW. If the speed jump is smaller, as in the previous diagram between fourth and fifth gear needs to be shifted earlier. Shifting out of gear doesn't always work. It depends on the diagram.