Hello!
You must first identify if the speedometer you see is correct, since there are two if you see the game's help system: "small map on the right side of your screen" within that window, there are 2 speedometers, the one that appears in a White circle "on the map" is the maximum speed that is set for that particular road.
While there is another speedometer in that same window on the upper left side, it is the one that indicates the real speed of the truck and therefore marks the speed at which you are driving.
Now also the feeling of speed depends on the loads and the truck you drive and / or the game settings you have.