Marked a line on the cement and another line on the front tire with a piece of chalk, got on the bike, lined up the two lines and rolled forward one revolution and made another chalk mark on the cement.
Measured the distance between the chalk marks and then plugged that into the cateye.
(the size chart in the cateye manual was off by 12mm)
Google maps said today's route was 32.4 miles, the cateye said 32.3 miles.
Close enough.