Speedometer trouble

[trunnell@mindspring.com]

Hi all,

Yesterday my speedo started acting funny, the needle was vibrating and reading 
erratically.  i dissasembled and cleaned it last night and it works smoothly 
now but is 10 - 15 mph slow.  I didn't see any adjustment mechanism.  Does 
anyone know how these things are calibrated?

Thanks,
Glenn