The Smiths Classic Speedometer has a handy drive-to-set function that makes setting the unit up very straight forward.
Owners may set their Speedos using a GPS application on their mobile phones to give a measured mile (or kilometer) or by using a mapping source such as Google maps.
You might be very lucky and have a "measured mile" on a local road nearby.
It is also possible to use Google maps to give you an approx mile (or km) or simply drive another vehicle after zeroing the odometer until 1 mile (or km) is registered, stop the vehicle at this point and make a note of the location to return to later on your bike or vehicle fitted with your new Smiths speedo.

Make sure the speedo is registering a signal from either an installed sender or from a dedicated ecu signal
Prepare yourself and your vehicle as normal for highway use.
Before switching on the ignition, hold the toggle switch in the closed position, if no toggle switch is being used simply contact and hold the Brown/slate (or sometimes just Brown) wire to a negative(-) source.
Switch on the vehicle ignition
The speedo needle will sweep across it's range and return to zero, now release the toggle switch (or the contacted wire)
The display will initially read SETPPU, press the toggle switch (or contact the wire) again until the display reads DTS PPU, now depress the toggle switch and hold for approx 3 secs and release, the display will show 00000.
(If the display returns to SETPPU when the engine is started then repeat the process above until the display reads DTSPPU and then 00000)
Ride the bike over the measured mile (or KM), at the designated measured distance bring the bike to a halt but leave the engine and ignition on
Depress the toggle switch (or contact the wire) for approx 3 secs and release.
The display will read DONE and then momentarily revert to SET PPU
You should now turn off your ignition to complete the set-up, The Smiths Classic speedo is now calibrated for either MPH or KMH, depending on the measured distance covered.

The Smiths digital range of speedos also provides a manual process for inputting the wheel data to calculate road speed, the instruction sheets (bottom of page) provide a full explanation of the "PPU" programming procedure. Press on the pictures below for an introduction to the process, the PPU setup may also be used to change the sensitivity of the speedo to verify sender operation.

sender setup and PPU function