How to adjut this code to set it to a fix movement speed (spline/timeline)

It’s like this:

Set it up so that you Duration is set to 1 and fiddle with your spline until you’re happy with the speed the car goes. It will probably quite a short spline at the moment.

You find out how long your spline actually is like this:

Let’s say it’s 100. I don’t know it might be 5000 or 3, it doesn’t matter, but let’s say it’s 100.

So you’re happy with your car going 100 units in 1 second.

Then add some more points to your spline. It’s now longer.

While the game is running now, you can calculate the Duration variable like this:

( see the 100 in there, that’s the 100 from above, but in your case it will be another number ).