Unsure if this is electrical or mechanical. Electronic speedo now reading vastly out, only really reads upto about 40mph, regardless of actual speed. Not had chance to have a proper look, any guesses at speedo or other end as pis sibyls cause??

Not sure what type you have but check magnets on prop shaft. If ones come off then this could cause a problem.

I've just changed my speed sender - I did have a 2 wire reed switch and now swapped it with a 3 wire switch. Before, the speedo used to jump around all over the place - now it's solid as rock!
So, it could be a knackered sender ?

Is it reading correctly up to 40mph?

I had a problem with mine before where the bracket that held the sender had moved slightly away from the prop.
Was fine at lower speeds, but as soon as i got to higher speeds, just dropped to zero.

Turned out the sender was too far from the magnet and wasn't getting a good reading

Pekwah ah, it's not reading correctly at any speed, just shows a maximum of around 40 regardless of how fast you are actually going.

[Edited on 6/5/14 by chrisxr2]

what speedo and sender are you using sir?

Need to go and check, bought car built.