User creating a content pack cannot set the acceleration, deceleration, and brake speeds. The formulas for drag and etc seem to only apply to airplanes, not cars and motorcycles and whatnot.

I've been working on this issue a bit in this thread: http://www.minecraft-smp.de/e107_plugins/forum/forum_viewtopic.php?43279 I can try to finish it, but I'm not sure how to get KeyInputHandler.java to constantly loop checking to see if a button is actively being depressed. There's a response to a key being hit but not being held down. I think I have the code ready to respond to such an event. But I don't know how to make the event.

I think we have updateKeyHeldState(int key, boolean keyDown) in IControllable, an interface of EntityDriveable