Profile: Speedograph Richfield Ltd. specializes in offering automotive accessories. Our products include angle drives, brake cables, control cables, corrector gearboxes, instrument fittings, motorcycle ammeters & cables and speedometer cables. The BG4604 corrector gearbox is made of a two piece alluminum housing providing maximum protection against water/dirt ingress. The input into the box is by a spindle connected to a gear train and output is through a second spindle opposite the input. These are normally fitted into heavy-duty cables to correct the calibration of speedometers and revolution indicators.