I finally got a computer for my bike, and it says my maps.google distance estimate is off by less than one-tenth of a mile.

The computer says I was moving for 1:13:56, while the stopwatch on my watch says the elapsed time was 1:16:14; so I only came to a stop for 3 minutes during the 18 miles, for stop lights.

Note: the computer says my max speed was 49.8 mph but I can't figure where I would have gone that fast. The fastest I observed was 27+ mph. The computer says my average speed was 14.7 mph.