1969 bsa a65

[marty 31]

my bike is now on the road and running well, my only niggle is the speedo, it shows 80mph when its doing maybe 50 mph, the clock is defo in mph, the drive I think is original to the bike, the drive and rear sprockets have been checked and are as the book, the drive as far as I can see has not got the 20 to one stamped on it, that the book suggests, is there any other way to check?

[L.A.B.]

the drive as far as I can see has not got the 20 to one stamped on it, that the book suggests

2.0:1  (not 20:1).

Where did you look, as the ratio and also the drive gearbox number is normally stamped on the inner ring so you'd have to remove it? 

is there any other way to check?

ACCURATELY count the number of wheel rotations to output (cable drive) rotations. If exactly 1 wheel turn to 2 output then it's 2:1.
 

However, the speedo ratio also has to match the drive gearbox ratio (and direction of cable rotation).

At the very bottom of the dial, there should be the speedo model number (SSM 5001/05 ?) and either '1600' or '1000' (cable turns per mile)?

A '1000' MPH speedo needs a 1.25:1 (15/12) ratio drive gearbox.
'1600' MPH needs 2:1.