So OddBot has posted some very interesting code on his service droid page: http://letsmakerobots.com/node/39492 (see downloads revision 2 code. This should give better speed control of the motors. I have a Rover 5 with 4 motors/encoders and mecanum wheels. The wheels don't work very well, so I want the best possible speed control of the motors. I would like to convert OddBots code so it works on my ATmega1280.
Now I don't really understand how the code works... I'm not going to post the full code, it's a bit to long. But this part is the most interesting: