80 B speedo inaccurate

[Gordon Bird <gb@the-bdc.com>]

I know that I don't have the orig engine, so I strongly suspect not the orig
trans, but the speedo reads slow, shows 60 mph when traveling @ 80.
Anything obvious to look for?  I recall many years ago about a 'reducer'
that, when inserted between cable and trans would reduce your speed/odometer
reading.  Just wondering if my car orig had OD trans, would use of non-OD
cause this type of error?
TIA

Safety Fast!
Gordie Bird
62 MGA
80 MGB
86 Audi 4kq