CABI stats: a mysterious difference in mileage

I think CaBi just guesses based on your start and end point. Check Google Maps bike directions for those two routes – it wouldn’t surprise me if the mileage matched up to the default routes.

IIRC, buried somewhere in their FAQ, it seems that they assume you ride at a constant rate (about 8 miles an hour, I think? Maybe less? I’ll have to look later), and guess that you went at that speed for the whole time the bike was out. Their estimated distance is a function of elapsed time, not station-to-station distance.

That formula seems close but not exactly what I found in a recent trip. I did a longer CaBi ride over the weekend, where I docked and undocked the bike at a few different stations. At one point, I undocked from a station, rode around for a while and docked back at the same exact station. That segment was 24 min. 25 sec. The distance was calculated as 3.26 miles.

If I punch in the numbers using the 7.456 mi/h estimate, I would get 3.034 miles.

It doesn’t really matter to me though. If I want to know the estimated distance, I bring my Garmin GPS watch. (I have the Forerunner 220, which I can use for running or on bikeshare.) GPS isn’t exact either, but it’s close enough for my purposes. I look at the CaBi distance, calories burned, gas saved and CO2 offset as very rough estimates. Some people might use them as motivation for getting out there more often. If so, that’s good. But I wouldn’t use those numbers to track anything precise. For example, if someone is tracking calories burned and attempting to match them up with calories consumed, the CaBi numbers aren’t a very reliable way to do this.

Other calorie counters are also imprecise, I think. When I load my Garmin data to the SportTracks program on my computer, I can click on the button and see an estimate of the calories burned for the session (whether it’s cycling or something else). I think the program calculates the number based on my bodyweight, the speed of the session and the duration. But I still don’t trust the numbers completely. It would be nice if the numbers were within 20 percent of the true figure, but if isn’t, I wouldn’t notice.

If I’m trying to lose some of the extra winter fat, then I know that I just have to be more consistent about exercise, and that longer workouts seem to have more of an effect on that. I also monitor myself to make sure I don’t overdo it, because once you get injured, your whole fitness program goes out the window.