Ticking Speedo

[BenGirling]

My speedo takes it signal from a Ford electronic sender plugged into the gearbox (type 9). Registering speed it initially jumps from 0 to 20 mph and then 'ticks' round rather than sweeps as speed increases.

I have completely rewired the car as part of a rebuild and using a different Caterham wiring diagram initially fitted a 1k ohm resistor into the back of the speedo - this served to kill the signal completely and the speedo failed to register at all.

Having now removed the resistor the speedo registers but behaves as described above.

I wonder if there is anything I can do to 'smooth' the signal or if this is how the speedo behaves?  The sender gives 8 pulses per revolution and I believe I have correctly set the speedo using the calibration calculation on the CAI website.

Thanks for any advice.