If someone switched out their factory guages for a set of tha white faced 200mph guages made by companies like BBK, how would you calibrate it right? Do they make a gear for that type of conversion??
Speedos are calibrated internally. They all count the number of times the cable turns and convert that to the speedo reading. They all read the same number of turns at the same speed. So as long as the speedo gear in the tranny matches the rear end gear, all speedos read the same speeds... 85, 140, 200 all read 60 MPH at 60 MPH if they are calibrated correctly in the speedo itself. You only need to change the speedo gear if you change rear end gears or change the tranny to a different drive gear.
For the 140 & 200 faces, Brothers does recalibrate the speedo for you like was said above.