I’m working off of the assumption that you are your project is based on the vehicle template. Inside the content browser if you open the Sedan folder that is a physical material named Sedan_PhysMat. There is a section named Vehicles with the option for Tire Friction Scale
It appears that the simplest way to control the coasting effect you’re looking for would be to set it up inside of blueprints. Depending on how you are controlling your acceleration you should be able to set up a timeline that decreases your speed over time. If no input is used to increase speed during this time then you can set the vehicle to come to a stop once the speed is reduced below whatever threshold works for your case.
I’m not sure I understand your question. When you take the current speed and subtract x, you should be able to set that result back into the current speed. Over time this will continually decrease speed and slow down the vehicle.
Hopefully this helps answer your question, otherwise if you could add more detail to explain what effect you’re trying to get.
Why do you want coasting from vehicle while several experts seem it as a bad practice? While driving at the sloppy way ,coasting is good for saving small amount of fuel but it has another side too. You don’t have proper control over your vehicle, if friction between car wheels and ground surface is less than a certain level during coasting.