So, what are members using for bar mounted speedometers ? I attempted to mount a bicycle speedometer made by Cat Eye, but the wire to the wheel was too short. I would like a gps unit, but they seem high priced and I don't know which are the good ones.  Any ideas ?

An Intruder speedometer is easy to adapt if your risers are tall enough to clear the indicator light panel. And you get a trip odometer as a bonus.
The cable is easy too. The tach cable for the old water buffalo fits and is long enough to reach.

I have a Garmin GPS (automotive type). It gives me the speed accurately (within 1 mph), and it does not matter what modifications I do to the powertrain/wheels/tires.

 I'd go with GPS, they can be found really cheap used and you get more than an accurate speed reading.  They will show trip data, averages, altitude, traffic, directions etc.  More bang for the buck in my opinion than reworking a front fork system.

I prefer the Vapor by Trail Tech.  My wife runs one on each of her bikes.  Take a look about half way down this thread page and you will see them from the cockpit as well as from the side.

http://suzukisavage.com/cgi-bin/YaBB.pl?num=1453341892/15

I also run one on Cafe a la Carte.  

I like them because they have all  the basics you need and fully programmable.  I just would not recommend it if you are determined to have a tach.  I don't care for their tach.

https://www.trailtech.net/vapor

You can find them very reasonable on ebay.  Just make sure you do your research and figure out what kind of mount you want for your magnetic pick up.  That will then determine the kit to buy.