I tried it again and this time instead of stopping the recording by pressing the record button I pressed the X on the top right.
Now speed is zero for the whole drive and g-force doesn't change (although it's not zero.)

I am almost certain speed and g-force were shown on the overlay while I was ...

I manually select video, then press the record button. Video records well and overlay works but the overlaid data (speed, g-force, etc.) is approximately 30 seconds behind the video. I'll be stopped and it will show me at 50mph for a bit. I'm running it on an iphone4 with no other apps running. I ...