Muskrat,
The speedo ratio selected depends on the sprockets selected. And the layshaft gear/sleeve gear. And the layshaft/speedo drive gearset.
I think you will find that you are reading slower than your actual speed if you have the standard set up.
With a 2:1 speedo drive the cable revs will be approx 1600 per mile and at that speed you will be doing 60 mph indicated.
However, with standard gearing 60 mph is reached at 1450 cable revs per mile.
Having said all that, my mate has an Indian made replica on a Flash. Before he fitted it he checked the calibration by driving the head at a known speed.
It was close enough to correct for the Flash gearing.
Makes you wonder how well these speedos are calibrated at the factory.
Off to my young days which is a long time ago, I fitted a Triumph "revulator" type speedo head to my Flash, blissfully ignorant to the different ratios.
The gentleman in a blue uniform driving a Cooper "S" soon set me right.
Trev.