Speedo steady but under reading on GAA12K

Sorted all electrical glitches and had Powerspark Electronic ignition fitted, car came back to life! :D

On road testing all seemed fine but speedo was reading very low - i'd guess 40mph at about 60, 20 at a real 30. :?

Car has 5 speed conversion, probably relevant. Does the car need some adapter or special speedo mech? Where does the speedo drive come from?




Well-Known Member
Staff member
Hi Ian,

The drive comes off the gearbox, so given the very large under reading of your speedo, that is the probable cause. The other would be running tyres or wheel/tyre combination with a rolling diameter far above the original size.


Your 5 speed box is the reason your speedo under reads. Ours read 2/3 of actual road speed when first converted so your estimation looks spot on. There are various companies who will recalibrate it for you.

Your mileage reading will also being be going up at 2/3 the actual rate as well, you might see that as less of a problem :D

Thanks all,

Any suggestions re speedo recalibration companies in UK?

Or can you get an adaptor for the speedo or gearbox end to sort this?


We had Speedy Cables make up a cable to match our 5 speed box to our standard speedo, they were very quick and the cable was perfect. They advertise that they do calibration as well so they are probably worth a try, our speedo still under-reads but that really doesn't bother us much with the way we use our car :D