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Ok, odometer play time.

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I couldn't quite tell about that side - your description makes sense though.

One thing I learned in 20 years of installing & servicing magnetic sensors was that you should avoid direct contact of the magnet to steel. It's way better if you can use a magnet that is already encased in plastic, as it bleeds out slower.
Your magnet stays stronger longer.
 
I used JB Quick because I'm impatient and it has held for three years so far.
You can get very strong rare-earth magnets from a bikeshop that are much stronger than the original, that helps. If you embed the magnet in a puddle of JB Qucik on the lip of the drum, it also keeps the magnet from direct contact with the drum.

Glen
 
MAgnet doesn't need to be all that strong. Depending on the sensitivity of the switch and again I found a switch at a local electronic surplus place, you don't need a big hunking magnet. Just needs to be < 1/2" away from Magnet to reed switch as a contact distance.

Make your switch mount adjustable, I simply used a right angle corner brace attached to a 6" approx piece of 1/2" flat bar stock that I drilled and attached to one of the U Bolts that holds on the axle to the spring. Just added a second nut to hold it on the existing U Bolt. The corner brace is drilled so I can more the magnet closer or further away from brake drum to adjust for sensitivity. Just set up a way to measure open/closed on the reed switch with a multitester so you can set it up properly. It will save loads of time in testing. I used flexible telephone line mounting cord and curly cord I had in my junk box. Wire is close to correct gauge, had jacks on end already I could adapt to make a test rig for checking open / closed. And as a member of the "we tight club" something I already had on hand.

For the dash mount, I took a small piece of spare sheet metal approx 4" long by 1" wide with a little 1" tab on the end. Think of an L shape. One end of the L attaches to the Windshield Rod Brace and the other end is bent up at 60-75 Degree angle. Cyclometer attached to the tab that is bent up with the rubber bands that normally attach it asround the handlebar. Painted it black, sits up on top of dash just on the other side of the padded dashboard top. Easy to bed into shape to get the proper angle. Does not obscure vision and I can see out of the corner of my eye without taking eyes off of the road.
 
Magnet glued to corner brace with a combo of zip ties and JB Weld. Corner Brace bolted to 1/2" flat strap. Loosen screws to adjust distances/clearance/sensitivity.
 
I got mine installed over the weekend, took it out on a couple short runs calibrating. Now I will fill the tank today and start to see what my mileage is like. Hopefully not too bad, but if so I'll know I have work to do.
 
So far I'm very impressed.
I need to go out with another car so I can check mph - or get a reading from the other car on distance from point A to point B so I can make sure mine says the same. At first I figured the number at 1112 but that was too high, I measured mileage between two mileposts and adjusted so I am pretty sure it's close.

The thing just works, wasn't difficult to install, doesn't look bad.

I used the sticky back and attached it to my courtesy light, the wire goes up over the top of the console & isn't visible.

I attached the switch & magnet by filing / sanding clean the drum & the backing plate and then 'glued' them on with Bondo!
After the Bondo hardened I painted everything with some black Rustoleum.
It will be interesting to see how long this lasts.
 
Bondo may let go as it is a fairly soft polyester material. If it does, that's no big deal, just find a better magnet like Jim suggested and bond it to the drum with JB Weld or similar. Which model bike computer did you end up buying?

Another method you can use to check your accuracy is to use maps.google or one of the other online mapping resources. Ask for directions from your house to some distant point. Reset the bike computer's trip odometer before you leave and check its reading after you reach the destination. Compare the trip mileage displayed to the online map distance. The actual speed is based on the scale factor so if you get the odometer/trip to match the online map results, the speedometer will be equally close.
 
Doug - great idea! I'll do that.
 
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