Speedo issues

[Den13]

Hi all having problems with my mrs speedo, it seams to be off quite a bit when doing 0-40 it's out Mibi 0-10 mph on speed increase then after that 10-20 mph from 40-100 anybody know what is causing this,is there a sensor somewhere I could change thanks

 

[stewd]

Mine says 35 when doing 30 and 46 when doing 40 and 78-80 when doing 70... Think it's just generally out... Just stick with what the GPS says.

 

[Den13]

Yeah but we don't have the sat nav stuck to the window all the time very rarely, it's a right pain

 

[Den13]

And mine says 90when it's doing 70

 

[stewd]

What tire's do you have on..?? (Size) as you can slightly adjust the speedo depending on your tire sizes etc... As depending on the tire size depends on how many times the wheel spins which then effects the speedo etc...

 

[Den13]

It's just 15 or 16 with big tyres as far as I know they are original to the car

 

[stewd]

Um... Weird... I would say live with it but if its reading 90 when you doing 70 that's way off I would say it should be 3 - 5 mph out. Check your tire pressures are up to what they should be.

 

[Den13]

Yup everything is tip top the way it's ment to be so surly there is a sensor somewhere I can change or clean or shout abuse at and see if it changes anything

 

[renton]

They is a speed sensor in the back of the gearbox. Try changing that

 

[Den13]

Ok will do hopefully solves the problem

 

[RobinJI]

I can't see how the speed sensor could cause it to be out a bit, it's just a pulsed signal, a dodgy sensor couldn't change the timing of the pulses it sends. It might miss pulses or stop them all together, but that'd cause the speedo to stop or under-read, not over-read. Still worth checking out though, it's quite simple and all just unclips with no tools if I remember right. It's on top of the diff and on a 1.8t (and maybe others) it's covered with a little heat shield pocket.

It does sound quite likely that the tyre size is out. What actual size are they? Seems odd though, being that far out they'd be quite noticeably visually smaller. Still there's a lot of room for error with a small wheel and big tyre.

 

[Den13]

Will have to look at tyre size it's the mrs car but it could well be, because I think the car came from an old dear she maybe took it to a garage and they put the wrong tyres on as they do seam quite new so might very well be that

 

[Den13]

ok so washed the a3 today a looked at tyre sizes thay are 195/65/15 all round sound off to anybody?

 

[stewd]

They seem a bit small to me, let me just check mine..

 

[stewd]

Nope.. Exactly the same as mine. Have you checked the psi in them all?

*edit

its also an eu law that the speedo has to read 3mph more than what you are doing.

 

[Den13]

Yep all the psi is the same and sitting at the right pressure, and mine is well above the law then lol ****** pain in the **** just want to know what's causing it

 

[stewd]

Where does the speedo sit when your are sitting still?

 

[Den13]

Erm 0mph then as you increase the speed it gets more and more out, I thought perhaps the clocks had been changed because is a 1.6 and the clocks got to 160mph I thought they might only go to 130 or 140 but checked 1.6 on eBay and saw pics of 1.6 with clocks that went to 160mph so kind of ruled that out,but at looking at mot certificate in 14 month the car done 1200 miles????? But the mrs says it was an old dear that did not work that had it but I still find it a bit odd(btw she picked the car herself I had no say in it......she knows best as always )

 

[stewd]

The dash cluster is the same as the 1.9, 1200 in 14 months for someone who just used it to go to the shops and back is reasonably at a push.. Reset your mileage counter and drive a mile according to the satnav and see what comes up on the cars counter, as could be an underlying issue somewhere.

 

[Den13]

Set sat nav up the other day when we went to Glasgow and it ok to maybe 10 mph then after that it get increasingly worse till you reach 90mph on speedo and your actually doing 70mph

 

[Birdy]

i woundnt worry to much if i was you. police allow 10% difference in speed for this reason no two speedos read the same. my work van has a gps traker an reads 70 when my van speedo reads 80ish just one of them things