If my Sigma Rox 6.0 is any indication, they're finicky to set up for exact measurements. If the world will not end if you did 8.0 miles or 8.2, then any of those would be fine. Otherwise a basic GPS box or smartphone app would be more accurate.

On second thought, do you really want to know you're going downhill at 8 mph on 2nd gear with a 20mph / gusts to 29mph headwind?