My lovely wife recently got one of those them there iphones. It's really neat, like a little computer and internet machine with a camera that also happens to make cell phone calls. Some friends use their cell phones to track and post athletic endeavors to the internet, including runs, walks, and bike rides. Since I am a coattail rider and follower, last Sunday I "borrowed" my wife's iphone, downloaded the "MapMyRide" app and used it on a ride, along with my bike computer to see how they matched up.
The bike computer finished up at 28.266 miles. The GPS "MapMyRide" said 29.04 miles. This is about a 2.74% difference.
WHO CAN I TRUST?
If I trust the GPS, then I'll need to adjust the calibration on my bike computer, which will change my future records in either a positive or negative way (in relation to distance and speed), but when I think about which way it'll be affected I get confused and move on. But if I trust the bike computer, I am basically admitting that I don't trust modern technology.
I like the bike computer better, as it doesn't record time I'm stopped. The "MapMyRide" app kept the timer ticking even when I stopped for a snack. So it registered my average speed as 15.8 miles per hour, while the bike computer showed 16.4, which is faster and thus better. My bike computer also tracks a top speed, which is a nice feature the iphone app did not seem to have.
The iphone had to be carried in my back pocket since I don't have an harness to attach it to the mike anywhere, so it got all damp and humid in the pocket there from the physical expressions of my exertion, and that wasn't pleasant. But the "MapMyRide" did the altimeter thing, and that was cool, too.
I think one day this weekend I'll use the iphone again and go on a large hill near my house to see if that qualifies as a "big climb." the "MapMyRide " won't classify anything as a climb unless it's 500 meters at least, with a grade of some degree that I don't know if I'd recognize it if I saw it.
Please let me know your opinion in the comments about your experiences and what you think of the truthiness of the GPS mileage data vs. the regular bike computer/magnet on the wheel setup. I suppose the GPS/iphone app should be more reliable than my programmed bike computer, and if the GPS is more accurate, I guess eventually I'll reprogram the bike computer to indicate a larger wheel circumference such that it will register a slightly longer distance with each turn of the wheel. I guess take the current circumference of 2091mm and increase it by about 2.74%.