Switching from Km, to MPH.

AJ/FormS said
But there are not enough pinions to make this switch using the MPH speed-o

Op has a KPH speedo he needs to slow the input down to make it read 60 when head going 60 mph.

I think the gizmo you posted would be a great way to go if a proper tooth pinion is not available.

AJ/FormS said
The ratio is .62

Multiply by 0.62 or divide by 1.609 same thing either way you are slowing the input speedometer shaft