Believe it or not but it is possible to adjust the speedo and tacho to give the correct readings. Details of this can be found in a very good book "Magnetic Speedometer Repair" by Graham Bligh. This explains how to rebuild and adjust the Smiths SSM speedo and tacho.
The tacho uses the same mechanism as the speedo to make the needle move. Both mechanisms rely on a magnetic disc spinning inside a metal cup to move the needle around the dial.
Inside both mechanisms there is an adjuster screw which varies the distance between the spinning magnet and cup. When turned, the adjuster varies the needle movement on the dial.
Apparantly, it is possible to adjust a 4:1 tacho so that it reads like a 3:1 and vise versa, although you would be at the limits of the adjustment to achieve either. You also need something driving the cable at a known RPM to make an accurate adjustment. The book suggests using a cordless drill coupled with a digital cycle speedo and magnetic trigger to obtain a known speed/rpm and use this for calibration of the speedo/tacho.
To make any adjustments, the bezel needs removing to gain access to the adjuster screw and its worthwhile drilling a small hole in bottom of the case so that future adjustments can be made without removal or disassembly. New bezels and glass are available on eBay
and can be simply siliconed in place for easy removal in future.
I rebuilt my A65 speedo using the book as a guide. It now works well and IM pleased with the result.
Hope this helps.