just use a watch. eddy merckx was the greatest racer of all time, all he used was a watch.

you know your route. you keep track of the time it takes you to complete the route.
you then divide miles by time to get average speed.

if your average speed was low one day, then you did not eat enuff carbs.
if your average speed was high but your legs were sore for three days, you need to eat more protein.

contacts? that's what i wear to see.
contacts? they are also the people i actually encounter in the real word, the physical universe, the bike riders on the trail. which whom i communicate with face to face, without a smart phone.
as soon as i worry about my contacts, i may as well stock up at Cabellas and move off the grid somewhere near the canadian border and hide in a rabbit hole, having taken too much LSD as a youth or been beaten by my father which causes me to be a paranoid conservative anti-social trump supporting survivalist who refuses to get vaccinated because he does not trust the goverment. and thus jepordizes(sp-sorry alex!) the general public with selfish and delusional behavior cause by too many psyche meds and a nagging wife.