Speedo Over Reading

[White S3]

As the subject says, Speedo reads over the actual speed. It is made worse if there is a drain on the electrics i.e. fan, indicators, lights, windscreen wipers. Not too much of an issue you say, keeps me the right side of the Law but the kids drop me in it with the wife "Dad was doing 100!"

The real issue is the fact that the mileage is also clicking over at the accelerated rate too - recon it is +15-20% so I have a 99K car that has actually only done 85K.

Firstly, any ideas what might be causing it (other than an earth). Secondly, does anyone know where the speedo/mileage sensor(s) is/are located and where do they get earthed to the chassis? Finally can the sensors be replaced?

Thanks
White S3 (Mark)

[mrufcs1]

If the S3 has the ford type 9 gearbox, the drive is mechanical. So either the speedo is dodgy or you got the wrong cog in the gearbox. From memory there are two ratios available?

Do you think the revs are unusually high ? It maybe the engine/gearbox is doing more mileage than the car ! You maybe have the wrong diff ratio if it's been swapped or even the wrong size tyres ?

[lewdon]

My S2 with standard gearbox has electronic drive to the speedo from a sensor at the back of the gearbox. Dont know if this helps., as I am not sure how it could generate too many pulses to make it read fast.

[lewdon]

As it gets worse with wipers indicators etc it might be caused by a voltage drop due to a poor connection somewhere. .Might be worth checking voltage at green wire behind speedo. As wipers indicators etc come off different fuses it seems likely voltage drop may be occurring between battery and fuse box (yellow connector?). I would probably try running a temporary wire from battery directly to live on back of speedo. with another temporary wire to provide a clean earth back to the battery. If speedo then reads correctly then it would be a matter of checking the pd across connectors until you locate the problem in the supply or earth. A cheap check before you start replacing or repairing parts. But please bear in mind that I know nothing and cant be held responsible if it lets the lucas smoke out of the wiring.

Edited by lewdon on Sunday 1st July 22:30

[DamianS3]

This is pretty common.. mine was accurate but gained 20mph when i put the lights on.

Afaik the only way to fix this is a recon speedo from not so speedy cables.

I have the recon speedo with lcd display and use a crank sensor like the chim no problems since.

Cheers

Damian S3