To Disable it - Select EU2 as that doesn't have the Speed Limiter enabled on that server.
To Fully Disable it do the following:
1. Load ETS2 or ATS with the Trucker.MP Client.
2. When in the Sign In Screen Click the Cog in the Top Left Hand Corner
3. Go into the General Settings Tab and un-tick "Speed Limiter"
4. Load into the Server or Game and go to Options and Click on Gameplay
5. Scroll down to Truck Settings
6. Un-tick the Truck Speed Limiter Setting
And that is how you disable it and hopefully that will help your situation This will also work with ATS as well.
