I have a Bontrager 300 computer with duotrap but I usually also use the phone to track the ride I do.

The thing is that there is a significant disparity in distance done.

What is more accurate?
The computer is obviously set up to the correct wheel size.
I wonder if the GPS is just not very accurate due to a deficit in the signal at some times.

Phone GPS sucks pretty hard. For the best performance out of the computer, you need to do a tire roll-out (or three) rather than just rely on the wheel size. As far as which is more accurate, who knows?

I'd think the computer far more accurate. It's a simple algorithm for counting revolutions and then multiplying the circumference of the wheel, even if you do not do a rolling calibration the difference between a 23 and 25 mm tire is a mere 0.5%.

Depends on a couple of things:

1. How accurately you set the tire circumference in the bike computer. Most people get this wrong and that leads to significant error.

2. How long the ride is.

GPS, when in good coverage, can be accurate to within 13 feet. So if you did a long ride, your start would be known within 13' and so would your finish. If the error was at max worst case, that would then be 26'. If you did a 100' ride, that would be horrible accuracy. If you did a ride of 10 miles, it would insignificant (<0.05%).

Note that changing tires sizes can have a big impact on wheel circumference and distance accuracy.

GPS based Bike computers typically compute the wheel circumference over many wheel revolutions. I would guess they update that on occasion. Cycling computers without GPS, you have to set it and there is no check on that over time.

FWIW, I found I had entered my wheel circumference incorrectly when I compared the results given by my GPS based bike computer. So I'd go for the bike computer as accurate (or the gps in your phone) over distance.

J.

This is ridiculous. Accurate to within 13' at any given time does not mean a cumulative error of 26' on a ride - it's not a simple measurement of two end points, it's the path traveled between those points, which can zig-zag all over the place and cause significant error.

The inaccuracy on either is small enough for me not to care. We're measuring bike speed and distance here not planning a moon landing.

So a route with lots of left turns will be more inaccurate on GPS than a route that has more right turns.

Hey, who invited the voice of reason?

This is the good old accuracy vs. precision issue. So long as the device is consistent, I don't care if it's inaccurate-- off by a certain percentage on everything all the time-- so long as it's precise. Repeatability is paramount.

Who invited the scientist?

Exactly. Finally someone who gets it. Who cares if you are .2 miles off.

^ This is what it's all about. I don't have a bike computer, but use my Garmin 920XT. I also have a Forerunner 620. Both are very repeatable over sections I use just for this purpose. I've also been pleased with their accuracy when I've raced (running) on certified courses. Even with taking the shortest route I can, I've not come in shorter than the race distance, 5K, 10K etc. After all, we're talking about a tiny, consumer based device. +/- 3 meters for accuracy is all you should expect. Good enough for me.

You're also comparing a dedicated device vs a phone (as is mentioned in the OP). Poke through some rides logged on Strava with a phone - some of them are downright horrendous; many (most?) phones have GPS that's in such need of help that they use cell tower triangulation in an effort to pin down their location at any given time.

I'm having some issues about my GPS vs my bike computer as well. A couple of weeks ago I did 46 miles and my bike computer said i averaged 20.1 mph while my Strava said I only averaged 17.8 mph. That's a huge difference. Is anyone having this issue or is this pretty common?

Differences in average speed are usually due to how the average speed is calculated (whether they're using moving time or elapsed time [and stop lights, etc lower your average]). That said, Strava usually defaults to moving time and it's usually the higher because of that.

The Cateye Strada computers on my 29er and Gravel bike are right with the GPS on MapMyRide app/Android phone. I have some Cateye Micros on my road bikes and they are only off from the GPS by a couple of a hundreths of a mile after a ride. Not enough to whine about.

wat?

GPS is close enough for me, for distance and speed. Also, those things are nifty to be aware of, but not really important. The things that matter about a ride are: did I have a good time, get good pictures, was it a good workout. A bike computer can only help me with the last part, the most meaningful way it does that is my 20 minute power average and time in zones.

I see some pretty wide variations in AVG between the various GPS devices my group uses. Distance traveled seems pretty consistent and accurate. However I agree that none of this really matters.

That was for purposes of illustration. The simple case is a straight line.

Presumably your GPS is running continuously too on some frequent sample interval. I know mine does t just take a reading at the beginning and end. So there is error in there also from sampling interval and from some error at intwrmediate points. it would be stupid if the GPS sampling interval were not short with respect to the speed of a cyclist. Your statement implies that I think a GPS only samples at the beginning and end. That would obviously be ridiculous.

All that said, screw up the circumference measurement on yor bike computer and you'll screw up everything on that computer - speed, distance and all measurements ties to distance.

All in all, if the circumference is set properly and your gps is operating properly, it won't likely matter.

IDK ... my Samsung S4's GPS reports a slightly different distance on different runs - +/-2% - and generally the phone reports less distance than my computer (Cateye Solar), often by as much as 8%.

The Cateye reports about the same distance as Google Maps does. The Cateye, without fastidious setup, is more accurate than my phone.

Why is this important? I have some mileage goals that will be tough to reach this Summer, and a device that reports distances inaccurately can keep me from reaching it.

Sure, I know reaching my goals isn't a big deal in the scheme of things, and 8% is even less meaningful, but it's easy to get more accuracy than my phone provides, so I just keep my phone in my pocket with GPS turned off and use my 'puter.

MY statement is the problem?

Lol okay.

I think it has been covered, but to amplify a bit, the two methods of distance measurement are subject to completely different error problems.

A bike computer that uses wheel revolution data can be extremely accurate if it has accurate wheel circumference data. Any error in this input will be cause a corresponding error in distance traveled, but it will be a consistent error that has almost no trip-to-trip variance. With a little care, you should be able to measure wheel circumference very closely (within a fraction of a percent), but I've always found the wheel circumference data that comes with the computer to be fairly accurate as well.

The GPS odometer has a more subtle error problem - the "jitter" in how it computes position. The computer will compute distance by sampling its position at some sampling interval and adding up all the distance intervals to get total distance travelled. The problem is that GPS position has jitter errors that cause position to "jump around" a few feet in random directions so while you may be traveling in a more/less straight line, the computer sees you making little zig-zags that add distance. On a curvy route, the computer will "cut the corners" and estimate a distance that is less than the distance actually travelled. And finally, GPS signals are subject to electrical interference, poor geometry, and issues of signal blocking (tunnels, tree coverage, buildings, etc.) that may cause momentary glitches with huge position errors, perhaps tens of miles. If the computer thinks you momentarily travelled a mile off your route and returned a few seconds later, it could add two miles to the distance.

All these GPS errors can be somewhat compensated for by averaging and filtering software as well as adjusting the sampling interval based on signal condition. A bicycling GPS may be optimized to minimize these errors for typical bicycle speeds. How effective they are with these techniques, I don't know. The slow speed of bicycles makes the problem harder vs. cars since the positional errors are larger with respect to the distance and speeds of a bicycle.

My experience is that GPS odometers do a "fair job", but can't compare in accuracy to a decently calibrated bicycle computer which uses wheel revolution information.

- Mark

I often run both Strava and RideWithGPS on the same phone.

And they give results that frequently vary by a few percent. I've seen Strava create zigs and zags that just don't exist with my riding.

For Strava, some of the Garmin devices are only accurate to 5 seconds, which can create interesting results for short segments.

Anyway, I'd trust the wheel mounted induction device over the GPS device, if properly calibrated. But, as mentioned above, does it really matter?

I don't know about distance, but my very old cateye 7 and Strava are always within 0.3 mph, and usually with 0.1 mph. They both calculate based upon moving time.


This amount of difference is better than I expected, and well within my needs.

Pretty much true if you have the wheel speed sensor connected to your GPS computer as it would then use this for instances when a GPS signal is lost during operation. As in heavy tree coverage over a ride.