120 and 160 mph speedos use the same gear, 140mph clocks have a different gear. So if you had 120/160mph clocks and swapped in a 140mph speedo, the reading will be out.
Also if the rolling diameter of your wheels are vastly different to stock tyres, your speedo will also read wrong!
You can swap your old speedo into your new clocks, they are all compatible. Even across VDO and Motormeter made clocks, I have a VDO speedo in my motormeter clocks
