Another way, rather than getting your speedo recalibrated, is to find one with the correct tpm, for your application. Smiths seem to have made hundreds of different ones over the years.
For example, using Andys 13.25/6 ratio (2.2083) and say 600x16 tyres' with an unladen rolling circumference of 88.39" or 2.4568 yards.
1 Mile =1760 yards, 1760/2.4568 =716.379 wheel revs/mile, 716.379 x 2.2083 =1582 tpm. Laden tpm approximately +2 per cent = 1614 tpm
So a 1600 tpm speedo should be fairly accurate. (if I've got my figures right).
A quicker way of calculating is
unladen- 20160/ tyre height in inches x drive ratio (20160/28.15x2.2083 =1581.5)
laden- 20600/ tyre height in inches x drive ratio (20600/28.15x2.2083 =1616.0)
Typical Smiths speedo tpm-
Morrris MM ( not 1000) MG TD=1600. Morris 1000, Reliant Rialto/Robin, Scimitar, ,=1000. MGB manual pre 74 =1280 post 74 =1000. MG Midget pre 74 =1376 post 74 =1000.
Land Rover early SWB =1536 early Range Rover = 1152.
Please don't take any of this as gospel and please correct if wrong.