The absolute highest top speed for the AMX-40 is 56kmh. However, this can only be achieved with endgame upgrades, which are very hard to get, and it would be a waste using them on a low tier tank.
There is a tool, called tanks.gg, that can be used to calculate such things. Here is a link to tanks.gg that shows how to achieve that 56kmh top speed.
Using the tool, you can build your tank as you wish, and it tells you the exact stats. For example, you add the crew-skill Brothers in Arms to your crew and tanks.gg calculates what effect that skill will have on the performance of your tank during battle.
You can easily use the tool for yourself, it's easy to understand if you have basic knowledge of the game. Just select a tank and then add the modules, crew skills and consumables. The tool will calculate every important statistic for you, even the hidden ones you can not see in the game (f.e. gun-handling).