Engine braking will use engine electrical power to slow down the truck, while the retarder will use the transmission "power" to slow down the truck. When it comes to cruise control, it only activates the retarder, because using engine brake will turn off the cruise control itself - pretty much the same way the main/pedal brake works. akira_aki_. You can turn it off by going into Settings -> Gameplay -> Truck Speed Limiter -> Uncheck. Last edited by Unstable ; Mar 19, 2015 @ 6:56pm. #1. BCS Mar 19, 2015 @ 7:01pm. My guess is that you drive with the retarder on, that will slow down your truck a lot. i don't know what bottom you have set up for it. so that you have to look for in your VDUH.