Xj650 Seca RJ speedometers reading low MPH.

I decided to overhaul my speedometer and replace the scratched face on it. It was reading accurate speed and the odometer was also accurate.
I cut the plastic case open in order to clean and lube the mechanism . I first tested two spare speedos that I have accumulated and found that after road testing on the bike they read at least 20 to 30 kms per hour lower than the original one on the bike. I cut both of them open and viewed the internals and they appear to be identical internally to the speedo which reads accurately. I am trying to figure out why the spares don't read the same as the speedo on the bike.
I am wondering if the magnet that spins and pulls along the needle shaft wheel is different or weak in the two spare speedos.
Any suggestions would be appreciated.

 

if you figure it out, let me know. i don't have any spares and the one ON the bike reads slow. the odometer works accurate. just the speedo. when i first got the bike it read 30 mph on the freeway. last ride it was reading about 50. on that same stretch of freeway. speed limit is 65. i'm sure the cars were doing 70. and i was going with the flow.

 

Gummy.

The lubricant in these old instruments dries out and gums up, and you can't actually SEE the most critical spots. Read this carefully; http://xjbikes.com/Forums/viewtopic/t=15303.html I dissected an older-style speedo that had screwed-together frame members rather than 'swedged' so we could see the parts you can't normally see. You CAN, however, successfully flush and lube them, once you know what you're trying to lube.

When the "jewel" that the needle drum rides on and its support bushing (see the above-referenced thread) get all gummed up, it will cause the instrument to read slow. They simply need to be lubed in most cases.