Bike Speedometer Accuracy 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:

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.