Measuring running distance

I primarily use three methods to determine my running distance: gmap it afterwards by pointing and clicking the USATF site, measure using the Nike+ shoelace-mounted accelerometer, or GPS.

On Sunday's run, the GPS and Nike+ (I wore them both, just for this experiment) staying with 0.1 mi over about 5 miles, until I started some steep hills; then the GPS fell behind: seems reasonable, since my toe speeds were surely much higher than my map speed. However, it continued to fall behind on the return jog, perhaps related to more sidewalk-to-street weaving to avoid traffic, or a slower overall pace.

Gmaps 10.21 mi
Nike+ 10.84 mi
GPS 10.36 mi