No need for any mods if u just want to change the speed bar. DaveKelly is correct, but I'll be a little more specific.
From the main menu, go to the controls and scroll down until u see Speed Bar. By default there is no key binded to it, so you will have to assign one yourself.
Now when you enter the game, you can switch between KPH, MPH and Knots. It helps me a lot to switch to MPH, because I'm American and have used imperial measurements my entire life.