Speedo over AND underreading??!

[stats2909]

Hey eveyone, little bit confused by this one....

... recently drove down to Devon from London (great top down drive ) and noticed that the speedo was under-reading by quite a bit compared to my GPS/TomTom. Then on the way back I noticed the speedo was spot on for a bit then it started over reading?

Tyres havent been changed/inflated/deflated and recently its under-reading again????!!! Its a 1991 S3, dont mind it being not being totally right, but consistant woudld be nice lol

Any ideas anyone?

Thanks in advance
Stats

[ceejay73]

Hi Stats, are you sure its ya speedo and not ya tom tom telling fibs (I know the Tiv isnt always accurate but dont know how accurate the sat nav thingys are either).
Anyway just by way of a quick check, the gearing of your S should give (according to the book) 23.39mph per 1000rpm in top so take her for a run, 3000rpm should be around 70mph (in perfect world). That should give you an idea of your speedo calibration and if you remember those figures its easy to check anytime ya like.

Hope thats of some help,
Carl.

[Peter m]

It might be an electrical fault. I have just found out that my S2 speedo varies when I switch the heater fan on. 70mph with the fan off increases to 80mph with the fan on full !!!! Probably a dodgy connection somewhere.

[stats2909]

Thanks all, I have heard that the sat nav should be vey accurate, aprticularly on predictable roads (like motorways and not country lanes).

I'm suspecting wiring however as apparantly (so I'm told ) when the car gets over 100 the speedo can fluctuate widely between 140 and 100.

Anyone know of a suspect points in the loom to look first?

Cheers,
Matt (Stats)