[psy_wombats]

Almost certainly the gyroscope. I'd try to avoid dead reckoning as much as possible, just because the counters are quite 'messy,' as far as flexibility is concerned. I can see it set up to work just as well using an interrupt-driven timer, but I just like the gyro because you can use PID control to get more refined values. (And just tell the robot to turn until it's at an angle. Much easier!)