These threads are admittedly pointless, but I'm bored and don't race for a few weeks so here it goes. Did the following workout today: 2 miles in 10:00, 4 min jog, mile - 5:00, 2 min jog, mile - 5:00.
The workout was on a treadmill at 1% incline. It has felt accurate on slower runs and tempos, but there's always the chance it's up to 10 sec off per mile at faster speeds.