I bought my dad an expensive wireless speedometer for his birthday from bontrager, and even that didn't work. First ride down to the bike shop and back (a few miles at most) it said he had gone 60 miles. (And mph reading seemed reasonable while riding it was setup).

I just go wired now. Suppose that won't work for the "clean bikes lines" aesthetic but it's just a lack of hassle.

The battery finally died on one of my wired speedometers after about 10 years and I had to replace it...that's all the maintenance I've had to do and it's worked flawlessly otherwise.

I don't know why they can't get around to putting decent wireless into speedometers (they have it working with cell phones, wifi, etc) but they don't.