If you fit the 747 - and if it fits, it will be 3.5% different, (less accurate), than the 773.
i.e. the rear wheel on a standard R80G/S turns 773 times per kilometer and that is why it needs a W773.
The normal R80 rear wheel turns 747 times per kilometer
Don't ask me why they are different
The correct Speedo for a standard R80G/S is W773 or W1244
You should be able to pick one up for under US$100 - used
BM Speedos are not particularly accurate - if you want complete accuracy, use something else
R90S, R100RS Motorsport, R80, R100GS PD, R60/5, K1
Now Sold - R80ST, R80G/S Paris Dakar, R80GS Basic