Hmmm... I'm not sure I understand what MrGuest is saying, but I would definitely use time_step for things like speed. It does, unfortunately, get a lot more complicated when it comes to acceleration, especially when acceleration depends on current velocity -- these are more difficult to make framerate independent, but it can be made close enough.

As an example, the friction (and thus acceleration) of the cars' wheels in KarBOOM depend on the surface type, a graph, and the speed of the wheels relative to the surface beneath them. At a high framerate this gets re-calculated much more regularly, resulting in slight differences in how long a car can be turning at full speed before spinning out. This kind of issue could be removed completely by targeting a specific framerate and just having the whole game slow down if it gets lower than that (by not using time_step), but that'd be bad for players like Redeemer whose framerate drops below 20fps quite regularly because his computer is very slow (and because of that he'll get an early version of the next update which will include graphics options so he can tell me what kind of framerates he gets ).

Do you have a link to the thread you quoted from? Found it. "mouse_force" automatically accounts for framerate.

Jibb