Does anybody know of a quality wireless Speedometer/Odometer that mounts centrally on the stem, rather than on either side of the stem on the bars?

It's just an aesthetic thing...

I have had a lot of success with the Sigma line of Speedometers/Odometers. They make several wireless versions with varying levels of features. They are almost universally available at your LBS or online. All their bike computers are stem mountable.

SIGMA SPORT

Cateye Strada. I have two and on one of the bikes it is centrally mounted on the stem, the other on the handle bar.

sigma. You can rotate the mount 90 degrees with a screwdriver. It's how I have both of mine mounted. Don't know about wireless, I'm not a wireless fan, but they have wireless and I assume they'd support it there too.

Any Garmin, but that's probably not what you're looking for. They're $$$ and need to be recharged after a few rides because they use GPS.

Everyone with non-Garmin solutions, the OP wants this for aesthetic reasons; if you could post photos, that would probably be helpful. And if it's not helpful, it's an excuse to show off your bike.

Buy a GPS computer (Garmin/Lezyne/Wahoo/etc). Wireless computers are junk. They're very prone to interference from lights.
I had a Sigma, it was junk, constant interference issues. Bought a Garmin 510 and am much happier. The only wireless computers that aren't junk use digital transmission, and cost just as much as a GPS computer.

I made an adapter by wadding up some fiberglass between the stem and mount. It was just spur of the moment when I was in the middle of a project and I saw an old computer mount laying on the table, although half-assed you can't see it under the computer mount and I'm still using it.

Well, I agree with the latter, but IMO that's a reason to use a wired comp. I just don't see the appeal of wireless. It's not like running the wires is hard, and it's not like hiding them is difficult either. It takes like 5 minutes to install a wired comp. I have no use for GPS on my bike.

Cool, thanks guys, I will check out the Strada and Sigma. I was hoping the wireless models would be functional, since the lack of cables gives a clean look! I can live with a discreet cable though. I'm reluctant to go with the GPS models, since I don't currently need navigation help. Also aren't these devices relatively easy to steal? Would be unfortunate to lose an expensive one.

I agree with your feelings about the clean look of wireless. I have "never" experienced interference with such devices, ever. Not sure where the critics ride.

Until my wife and I started using our phones, paired with a Wahoo RFLKT, as bike computers, I was using a Cateye Strada Double Wireless and the wife was using a Sigma BC 16.12 STS CAD wireless. I too would like to know where all this talk about interference with lights came from because neither of us have ever had any interference issues from our bike lights or other electronic devices.

Just find some vinyl (electrical) tape that's the same color as your frame and it's easy to make the wire invisible from more than 5 feet away.

I got one of these : CatEye Strada Double Wireless Speed and Cadence Bicycle Computer CC-RD400DW
I see they are going for $66 on Amazon now.

I use the IPBike app (Android) on my phone combined with cateye cadence/speed sensors and a garmin heart rate monitor. I use a phone mount that takes the place of the riser cap. This totally replaces the Sigma Rox 9.0 that I used for years, and I couldn't be happier with the setup. It's essentially the functionality of a Garmin for a fraction of the price.

IPBike can be used standalone with GPS only, if your phone does not support the ANT+ protocol.