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What factors should one look for when purchasing a speedometer? Does wired vs wireless or anything else factor into its accuracy?

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marked as duplicate by Batman, Aaron, freiheit Apr 29 at 17:41

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Somehow related: bicycles.stackexchange.com/questions/5145/… –  Benedikt Bauer Apr 28 at 16:40
    
If it helps any - my Garmin device will use its inbuilt GPS chip to calculate speed. However, if I link a dedicated speed/cadence sensor to the device, it will calculate speed using this sensor instead. So possibly if you had a speedo based on GPS, it would not be as accurate as an old-fashioned magnet on the wheel. –  PeteH Apr 28 at 17:01
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Mainly careful setup determines speedometer accuracy. You need to accurately determine wheel diameter and feed that into the unit setup. If that's accurate and the unit's clock is accurate then speed will be accurate. –  Daniel R Hicks Apr 28 at 17:29
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Unless you increase the sampling frequency a lot GPS tends to underestimate distances, since people rarely ride in straight lines. I've seen it entirely skip a spiral down-ramp, for example. –  Mσᶎ Apr 29 at 4:47
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I think that my GPS (Garmin Oregon 450) had settings which enable you to change the frequency of recorded data points. I think there was an "auto" option which records more point when you're going faster, and fewer points when you aren't moving. Also, I assume that the "distance" traveled will vary depending on how you (or the GPS) interprets the data. If you go straight point to point, you'll always end up short. If you try to plot a continuous curve through the points, taking speed into account, you can probably get a much better estimate. –  Kibbee Apr 29 at 16:39

3 Answers 3

I don't think accuracy is a problem, even with cheap ones. I tested my first one (cheap chinese) against Google Earth and the marks on the road (every 100 m) on a paved straight road over a 15 km ride, and the differences are minimal.

Cheap models only let you specify the nominal wheel size in inches. Better ones present a list with all relevant wheel sizes (inches plus variants like 700c, etc). The best ones let you input the actual size of your wheel (you have to measure it).

Choose one with a good screen (high constrast, big digits). Cheap ones are illegible under not ideal circunstances, and some models are very easy to reset if you accidentally touch the wrong button (that happens to me some many times).

I don't like wireless, by the way: too many problems and no benefits.

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Most speedometers are accurate as long as you input/set the right wheel size!

I have used very cheap ones, and they allow me to set the wheel size in milimeters, and after checking the same path with two different cars, the difference is minimal compared to what I had with the bicycle (in a 25Km ride the difference was only 10 meters compared to the cars).

As for the wired and wireless, my opinion:

Wireless:

Advantages:

  • Less wires;
  • Portability (I can put it into another bicycle very easily, no wires to worry about).

Disadvantages:

  • The wireless that I've used (cheap ones) normally have interference caused by the road "radars" (I don't really know the proper name), or even my mobile phone (if I put it close to the receiver);
  • More batteries needed! (since the emmiter is not connected and needs to send the signal via radio, it needs another battery, so two in total).

Wired:

Advantages:

  • No interference
  • Less batteries needed

Disadvantages:

  • More wires (most are very long and I have to make a small "pack" with the remainder)
  • Less portable (I need to take all the wires and refit them into the new bike)

I prefer the wireless due to the less wires.

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I assume we're not talking about some low-price, no-name thing from a Walmart sale but about one from one of the more common speedometer selling companies (cyclosport, sigma, garmin, to name a few). With those, I don't think that there will be significant differences in accuracy.

Also wired vs. wireless should not make a difference by itself (unless the batteries of the transmitter get weak on a wireless one, of course). In former times (15 years ago or so), I had some issues with a wireless speedometer when crossing or riding right beside electrified railway tracks, where the transmission seemingly was confused by noise from the railroad's power line. Nowadays I would guess that wireless speedometers use digitally encoded signals so this issue should be gone.

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