Long Run Nick

Did a 70 mile birthday ride/after a 6.7 mile run: 67 today. A birthday brick! My bike computer reflected 70 miles on the button and my Garmin 71.92. I stopped the Garmin when I took a couple beverage breaks.

I have had Garmins since the old 101--the brick/a 305 and recently purchased the 110. I really love the 110. So much smaller. Can wear as a watch.

Look forward to your experience with GPS vs Bike computer. I have ridden with the 305 and the mileage always reflect a little more.

Thanks.
Nick
71,000 miles + running
2,700 on my Trek 7.3 hybrid since 5/5/10. No biking since 1998. Last running injury.
No swimming> I only like to get wet when I am out running/riding and when I take a shower.

eco

I've had a cheap cateye, to a sigma BC2209 to the edge 500 that I currently have. They all pretty much said the same thing based on a marked path I ride every once and a while.

I don't know if I would have noticed a difference of a mile or two across a 70 mile ride though. Doing the same route with the same computer on different days will probably give you that level of error anyway, so I never worried too much about it.

ProLiteNews

Depends if you have calibrated your computer properly.

If your computer is properly calibrated, I would trust it over a Garmin any day of the week.

To calibrate a computer, mark the ground where the valve is, then ride in a straight line and mark again at the point where the valve meets the ground. Stay weighted on the bike. Now measure the distance between the two points in mm. There should be a setting in the computer where you can tell it the distance.

I always check my computer afterwards on known and marked distances. I usually set up at a track and ride the white line straight around. When properly calibrated, I can do 20 laps with very little deviation. If you do a 1km track with less than 2m of deviation per lap, you are under 0.2% error level.

The computer works very simply. It just adds the distance represented by one revolution every time it receives a signal.

While it is somewhat prone to error over very long distances, if you set it up properly, it should be very accurate, within a fraction of a percent.

The Garmin will be more accurate for very long distances, and might even be able to work with plotted non-linear courses, but won't be as accurate with hilly courses.

My computer is usually within 0.2% accurate. Over 70 km, that's an inaccuracy margin of around 140m. That's around 0.1km. Your discrepancy is 1.92km, which is 20 times greater.

My father is a cartographer and helped develop some of the systems involved with mapping and GPS. We're a bit nutty for accuracy on stuff like this.

SBRDude

My Garmin Edge 305 seems pretty accurate, or if it's making mistakes it keeps repeating them. The ride I do most often is 23.75 miles and the only time it's more than 1 or 2 hundredths different is when I have turned it off longer/earlier at a stoplight with heavier than normal traffic. I have had a variety of regular bike computers over the years, but always felt they were a bit off and especially noticeable at long distances where even a fraction of a percent can mean a half mile off.

StanSeven

It would be exteremly rare for a cycling computer to be more accurate than GPS over something as long as 70 miles. GPA works by triangulating positions from at least three satellite positions. Individual positions can be off as much as ten meters from noise in the signal, weather, and obstructions. But over a distance like a mile, your GPS takes many, many readings and the individual variations quickly balance out.

Rogue Leader

The highlighted part is important. Is your trail hilly? Even a little hilly over 70 miles that could make a difference of a mile vs a GPS which doesn't take elevation into account when calculating your position. As long as your bike computer is calibrated properly there is no reason it would be less accurate than a GPS, and in reality should be more.

I like free

It comes to a question of accuracy vs precision. Your garmin is more accurate, the computer is likely more precise. I say this because the computer if not moved will measure the same distance every time. It may be a bit off though based on the location of the sensor on the wheel. The garmin on the other hand is more accurate, although it is not always repeatable. If you look at a garmin track log in a mapping program such as arcview you'll see thousands of waypoints instead of a line. So as long as you are getting a signal the gps is taking waypoints and calculating distance traveled. If you happen to make sharp turns or lose a sattelite the corners will get cut off and that measurement will not be calculated. That's why survey grade GPS's have limits you must set so that if you aren't getting a good reading you get a warning so you don't continue moving. So to answer your question it is likely that they both are wrong but the computer will be wrong the same amount everytime while the garmin will change.