when i was looking into speedo drive ratios it was a bit unclear what you actualy get
all sierras had diffrent wheel/tyre sizes, diff ratios and there is more than 1 type of cable drive in the back of the gearbox (type9).
i could not find out if all the dials are 1000rev/mile
there is more than 1 way to work it all out and im not upto date with the ford speedo pickups but didgitalspeedos one is 6pulce/rev and is reedswitch type.
you have to mesure around the wheel like you would your waist then work out how meny turns a wheel does in one mile then work out the wheel/cable drive ratio and work out how meny cable turns in one mile and how meny pulces your pickup has.