I still remember lusting after this one pretty girl in 1982...

...wait, I mean her Pacer 2000 bicycle computer, which had 6 buttons and amazing functions.

(OK, she was pretty cute too).

Some times, I'm still think that one of the biggest improvements in riding technology wasn't indexed shifting, but electronic bike speedometers. Those of us that had to estimate mileage via Multitos or cable-driven speedometers back in the early 80s can commiserate.

In the early 1990s, I standardized my bike fleet on an Avocet cyclometer. One head that could move to many different mounts on each bike. just quickly changing the calibration number each time from a handy list. When properly calibrated, accurate to within well less than 1% error.

But how do I calibrate? I can do a rough estimate using a rollout on the driveway (several revolutions, butt on the bike). But to get the final number, I use a calibration course that is 2.151 miles long from my mailbox to the stop line at a nearby collector street intersection.

But how do I know that mileage? By using my motor vehicles, which have distance measuring instruments connected to the ABS or transmission sensors - super-precise odometers & speedometers that are calibrated over several miles of known-good mileposts around the state. When properly calibrated and synchronized with good surveyed mileposts, it'll tick over .000 right at each milepost. In fact, I have to adjust periodically for tire wear. And I verified with more than one vehicle. And my wife likes them, because she can see from the right seat exactly how fast I am speeding driving in big numerals.

So now we have the wonder of GPS and 'smart' phones (I use RWGPS as our club has a group membership). How does it compare? Usually dead on, or differing by 0.1 mile depending on rounding, even with startup & stopping at intersections & destinations. Except if there is an extended on-foot break in the middle of the ride such as volunteer work, breakfast, etc. Then it's sometimes different, with the RWGPS invariably higher. Even though RWGPS pinky-swears that it is auto-paused and doesn't include that obviously-pedestrian-speed mileage in your riding... they often do include it in the final tally, even if they don't ding you for it on the average moving speed.

Am I overthinking this? Yes, and for almost 3 decades. And my carefully-hoarded pile of Avocet 45s (and other compatible ones) is ever-diminishing, so someday I may go all phone-based. So all I can say is: your mileage may vary.