Thanks Tuan, for all the pointers, here and on bullet forum.
I'm not sure the mass-ratio is strictly a FP precision thing.
If it was, then using doubles instead of floats would solve it.
Yeah, I never made my sims tuneable on the fly, but I did fantasize about hooking up a MIDI device for real time tweaks. Can you imagine turning a knob on this to change tyre/road friction, e.g.? http://images.junostatic.com/full/IS396005-01-02-BIG.jpg
Never got around to doing this, but I should.