What factors should one look for when purchasing a speedometer? Does wired vs wireless or anything else factor into its accuracy?
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:
- Less wires;
- Portability (I can put it into another bicycle very easily, no wires to worry about).
- 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).
- No interference
- Less batteries needed
- 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.
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.
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.