Hello Clock, is your mentioned timeline speed the playrate or the alpha comming from the timeline?
Really trying to understand this… So the Alpha Value from the timeline seems to be a number between 0 and 1 while constantly growing until it reaches 1 then starts again.
This grow effect happens to be faster and faster as I add more and more to the spline, therefore the car moves quicker and quicker.
But why does the alpha value gets even affected by the spline length?
Here is a short video, just to make sure we all understand whats going on 
[video]TrackSpeed.mp4 - Google Drive