Thomas
Given that you're 109 isn't standard, you can't be sure what the speedo is fitted as there are a number of different ones depending on original tyre size, or what drive is in the gearbox etc.
It's probably easier to get a passenger to sit in with a sat nav while you're driving and when it reaches 30, 40, 50 etc they could place those sticky dot things on the face of the speedo. You might be a couple of mph out either way, but it'll give you a near enough reading for road use. Given it's also ex military, you could check that the speedo head isn't in km/ph instead of MPH.
Duncan