In your 3580 you have the option to change the speed limit indicator on a particular road [and for speed segmented sections of long roads] by pressing the SL Indicator while in simulation. However, you will need to do that after each turn and for each new segment whenever the speed limit changes. If you don't reset it, the simulated speed will reduce back to the posted limit or below. But the most you can increase it to is 180kph i think, regardless of whether the SLI shows 10 or 120 kph.