This was already mentioned once in this thread: https://forum.simutrans.com/index.php?topic=17201
It also comes up in the German forums, so let's make a proper request:
I don't think speedbonus needs to be displayed as a percentage in the game. It should not matter to the player whether they can reach 100%, only whether they can make profit with their vehicles. As such, it would make sense if you would not choose the bonus or malus, but a vehicle type and it's speed to see how much you can earn with each good. I made a bad mockup with paint of what that could look like.
Internally, the logic stays exactly the same. Choosing vehicle and speed would be exactly the same as if you adjust the percentage in the current version until that speed is shown for that vehicle type.
If nothing else, at least it allows me to type in what I want, rather than having to adjust the number by clicking umpteen times. And by starting from speed, it could remove the doubt as to which is correct when multiple percentages give the same speed.
Indeed, it may be easier for people not creating pak sets ...
The good windows has been overhauled. Hope, that you like it.