Let's start with the obvious: if you have a Cortina speedo, gearbox, axle, and running standard wheels amd tyres then the calibration will be correct and the speedo should read normally. Of you've fitted a lower diff ratio the speedo underread but should show 50mph flat out even with a van diff.
As you're probably aware, the speedo needle is free to turn and driven round be magnetic force. Of it rises normally as you accelerate then stops abruptly at 30 this implies it is being physically stopped by an obstruction. Can you take the gauge glass off and see if the needle moves freely by finger?
If the speedo needle pulses back and for at 30 it could be that you need a new speedo cable
Last edited by y cymro; 24th July 2013 at 23:37..
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Speedo
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