Speedo question

With a 19" wheel and 3.25" tyre, that theoretically gives  803.5 wheel turns per mile, or 1607 turns per mile at the speedo if you have the standard 1:2 gearbox at the wheel.  If you have a 1450 speedo, it will theoretically read 11% fast according to my calculations.  So 10 miles on the road will read 11.1 miles on the speedo.   Is this what you have?

A 1600 turns per mile speedo should be very nearly exact.  And since it's illegal to under read, it should perhaps have a 1550 which would read 4% fast.

I thought most unmarked speedos were 1600.  I might be quite wrong on this!

I don't know if the speed reading really is accurate against the odometer, or if the speed reads a little faster (to be legal).  Can anyone confirm this?  But an accurate odometer needs a 1600 turns per mile speedo. 
 

As 19 " tyres are becoming hard to find and the wider tyres have less room at the front of  the swinging arm perhaps a switch to 18" would not only help  that but reduce the error from your incorrect speedo ?.  How does the Maths work out David?! . Also a reduced seat height would help get my Arthritic leg over the seat and help my stands to be more secure.Les of  Norville built a lowered 650 SS  for his daughter.

Smaller wheel means more turns per mile.
 

So with 18" wheels Andy's under reading speedo will  improve its accuracy ?. I know you like puzzles David !.

...he'll need one with a bigger count per mile...
The front wheel gearboxes came in two ratios.  Perhaps his speedo is earlier...
There are usually some for sale at jumbles.  But are those without numbers always 1600?

The 1959 Smiths Motor Cycle Equipment list the speedo for the Norton Model 99 as the following Speedometer. S.467/203/N. The speedo cable was 5ft 71/2 inches, and speedo gearbox was BG.5100/251. The speedo has this number stamped on the face of the speedo. This speedo was also used for 1959, on the Model 50, ES2, 88, and the 99. The Nomads used a different speedo designation number. 

Andy H. do you have access to a cycling type GPS device?   Comparing the GPS to the speedo would give you a good idea what's happening.

I did this with my bike, it has a 2:1 rather than a 19:10 speedo  gearbox.  Showed me the speedo and odometer were good up to ~50mph, above that not so good, fortunately in the "legal" direction.

If you can do this, do it over a reasonable distance, say 20 miles +.  This will help reduce any errors due to hills, break in line of sight and the relatively slow transient response of these devices.  Then process the data through one of the free apps, I use Strava.

= so as to reduce errors