Geez, REALLY!
When I cared, I would place the tire stem down directly over a line crossing my garage, then roll out, perpendicluar to the line, while leaning over the handlebars to weight the tire. Go two or three revolutions, mark the distance on the floor, and measure. Inches, mm, doesn't matter. Conversion from inches to mm is EXACT (to the limits of your measurers resolution, stretch, and thermal stability) at 25.4 mm per inch (per the international metric system conversions, see NIST, for example). And, yes, those distances were often different from the numbers included in with the cyclometer.
If I get into Rando, or similar, I may go back to that. Otherwise, when I care, an estimate from Google Maps is good enough.
As to GPS, don't neglect the uncertainties there. Mentioned above is the periodic distances between discrete points. Others include: the inherent uncertainty of the geographic fix (up to 10 METERS old school ((~33 feet), around a meter now) provided you've got a good satellite constellation, often questionable around tall buildings, deep forests, and percipitous mountains. I've seen errors over a 1/4 MILE under such conditions with survey-grade units! It's those uncertainties that make geocaching so much fun!
Cheers!