I display 'Avg Spd' on a simple computer ( the lightest I could find ) and eyeball it as I cross the finish line on TTs.
I do a quick mental calculation to see if the timekeeper's result agrees with my Avg Spd. Or the other way round
I do a quick approximation of any wind assistance / hindrance and compare my final result with the last ride on that course.
Carrying Garmins and / or Mobile phones is ADDING WEIGHT.
On the other end of the See Saw, I accept the time written on my Brevet Card when I finish an Audax.
To get an estimation of how well I've ridden, I use :-
( 25 kcals per km, + 25 kCals per 100 m climbing ) / time.
Its not absolute. Its a quick method of gauging my kCals/min exertion during a ride, and comparing it with past rides.