SPEEDO 40% FAST

[yzzidog]

Hi, all - happy new year and all that. I'm running an '85 cabbie, GTI Engineering tuned 1.8 into 2L/5-sp.
Love it. However - rev counter works fine but speedo's 40% fast - what's the best way to put this right? All MOT's are being tightened and I'm worried it might fail me. micky.black@mail.com 

[chalky white]

Is it anything to do with the speedo wormdrive in the gearbox end? Thes are different  colours and sizes. So would this explain. Plus are they the Origional clocks to the car or engine/box?

Just a couple of thoughts.

[SoundillusioN]

And just to add to the above, you haven't got tiny wheels on have you?

[yzzidog]

THanks for the feedback, guys, I'm tending to agree it's the worm-drive. Wheels are Castellet 15s.
All I have to do now is find someone capable of finding and fitting the right one...
HNY. M

[raferackstraw]

i think you will find no matter what drive you fit your speedo will never be 100%, you need to work out the tyre circumference of a standard wheel and tyre fitment, and then compare with the tyre fitted to your 15" wheel, and then try to get the worm drive that gives you the closest match. dont forget that all manufacturers build in a 5% over estimate into all speedos.
i tried for ages to get mine right with 15" wheels and the best i could achieve was correct at 30 and 50 but it started to read under by 5mph at 70 and it increased the higher your speed