Old Smiths speedos and I think also 60s Jaeger types have the number of turns per mile (TPM) marked in the lower edge of the dial -- usually a number between 880 and 1200.
As a guide
Ford and Triumph are generally 1000 tpm
BMC fwd 1025 (may vary)

For a speedo cable to mate Ford gearboxes to Smiths small screw type speedo heads look at 60s Fords. Cortina 1 & 2, Anglia Corsair.

Useful stuff Trident- thanks!

James