Speed units are connected with language and for some reason are unable to modify with mods now.
I am the creator of the corresponding mod and since it's not working, here is guide with easy steps to follow.
https://steamproxy.net/sharedfiles/filedetails/?id=1744399898

1. Open the folder with game (by default: C:\Program Files (x86)\Steam\steamapps\common\Mashinky\mods).
2. Open "mods" folder and there open "finished_texts.zip".
3. Open "config" and then open "text.xml" with notepad (or any app that can open it).
4. In this file you will find a lot of text. If your language is not english, then find <resourses> line where "caption" is your language (you can do it with Ctrl+F).
5. Press Ctrl+F, type "DC400F11" in search bar and press enter, or next, or down arrow depending on your app. You will find line of <string> and number "0.621371192". To change units to km/h type "1". To change name of unites change the line below with "mph" to "km/h".
(Actually you can type any coefficient and units you want to use.)
6. Save changes (Shift+S) and if you opened zip file with something like WinRar don't forget to save changes in open WinRar window.

If you have done every step then (i hope) you will find km/h in your game.

If you followed steps and didn't get km/h then write in the comments, i'll try to help.

Also, developers in some future probably will make switch to change units directly in game.