I think the four options (40 / 80 / 160 / 320 km/h) are OK and enough to be useful.
But it seems that the simulation speed is a little bit lower than the chosen speed setting. I made a test route with a length of 10km and stopped the time the simulation took from start to end.
With a simulation speed of 320km/h it lasted approx. 2:30min (should be approx. 1:50min at this speed).
With a simulation speed of 160km/h it lasted approx. 4:55min (should be approx. 3:45min at this speed).
Times were nearly (but not exactly) the same while using the smooth movement setting switched on or off.
So I would suggest to increase the simulated speeds about 30-40% for better corresponding the chosen speed.
And would it be possible to keep the “Skip previous / next turn instructions”-buttons active even while moving (to not being forced to use “pause” every time you want to “switch” forward)?