BSA has quite a few different "ratios". It looks to me that your ratio should have been 1440 and this is why with a 1620 ratio you have about 11.11% difference ...........
Thanks Ziv, I figured that out a while ago fairly much same calculation as Richards, before I put the bike back on the road so I'd know in advance. I've also fitted a digital bicycle speedo (which somehow went to the default calibration 203cm instead of 213cm, so caused my a speed fine!!!), and since then also a bicycle GPS speedo, and recalibrated the digital one.......they all now co-relate nicely with my ~11% 'errror'....
...I also had a digital one on the MotoG, but spilled brake fluid on it when changing the handle bar, and it dissolved
which prompted buying the GPS one (by chance the same day the overdue speed photo arrived in the mail= but that's another story). Handy thing about it is I can use it easily with whatever bike/vehicle.....no calibration required
So long as I know the 'error', and the speed zones which are in miles anyway, I can just get used to it- Or just look at the GPS in (speedo in tank is not always convenient anyway)