Hi folks,

 

Wonder if anyone has any advice?  My sister has recently bought her first car (2001 Fabia I PD100 Elegance) but we've noticed a huge discrepancy with the speedo today.  People are occasionally overtaking in built up areas which first alerted us to the issue.

 

Two sat navs are both reading 24mph with an indicated 30mph speed on the speedo.  I'm going to take it out on a motorway to check higher speed discrepancies soon.

 

But how do we resolve this?  I know the speedos can under read by up to 10%, but 20% is quite considerable.   Tyres on the front are 175 profile as opposed to 185 but they're 14" which are correct for the vehicle.  I've so far only come across topics referring to 5-8% under-readings with people suggesting getting larger wheels to compensate, but don't want to go down this route.

 

Any help as to what we could do to get this closer to the usual 10% or lower is greatly appreciated.  There are no fault codes relating to any sort of sensor that may be related.

 

Thanks in advance 

Edited 13 May, 201412 yr by Dave89

Yeah that's about right I had that in my previous mk1

what is the exact tyre size that it's running, width and profile? 175/?? r14.

What size does it say on the inside of the fuel cap?

http://kouki.co.uk/utilities/visual-tyre-size-calculator

 

175,  185, 195 etc are the Tyre Width side wall to sidewall, not even the tread width.

 

The Profile is the 55, 60, 65 etc,, Aspect Ratio.

the percentage height of the sidewall to the tyres width.

 

What your Sister needs on the cars is tyres with the correct Total Diameter/ Rolling Circumference.

ie

The right size for the car.

Yeah that's about right I had that in my previous mk1

 

Ahh... Wondering if it's just one of those Fabia things! 

 

My VRS is only about 4-5% out compared to this which is under reading significantly.  I doubt it will extend so much up into the 50/60/70mph range as confident the car seems to travel a similar speed to traffic.

 

I only noticed the tyre size issue when looking at causes.  

Fronts: 175 65 R14

Rears: 185 60 R14

 

Thanks all for you input so far.  I will recommend the tyres are matched to the rears regardless of the speedometer issue - but if it helps, then great!

 

i wouldnt trust your sat nav to give an accurate speed reading  

I would recommend that you recommend finding out which tyres are the correct size for the car,

then have the correct total diameter size on the front and rear,

 

dont go matching a pair to another, until you know what the correct size is.

I'd trust a sat nav more than the analogue clocks to be fair. With a good GPS signal they are usually accurate to within 2-3mph, though elevation can knock it out slightly on a flat road they are pretty accurate. More so than the speedometer anyhow.

 

Back on topic, I'd get the correct tyre size fitted. Could well be adding to the problem.

Thanks all.

 

I believe 185 60 R14 are correct, I will check the manual/fuel flap.

 

Both sat Navs (one iPhone, one satnav) read identically.  I recognise they both may be reading out but the speedo is definitely that little more out than I'd expect.  Hopefully tyres will resolve this.

 

Cheers

Thanks all.

 

I believe 185 60 R14 are correct, I will check the manual/fuel flap.

Cheers

 

Yep, that is correct:

 

http://www.briskoda.net/forums/topic/217351-oem-tyre-sizes-and-alternatives/

The speedo is "over reading" not under. It's showing more that it should, not less.

 

 

Ahh... Wondering if it's just one of those Fabia things! 

 

My VRS is only about 4-5% out compared to this which is under reading significantly.  I doubt it will extend so much up into the 50/60/70mph range as confident the car seems to travel a similar speed to traffic.

 

I only noticed the tyre size issue when looking at causes.  

Fronts: 175 65 R14

Rears: 185 60 R14

 

Thanks all for you input so far.  I will recommend the tyres are matched to the rears regardless of the speedometer issue - but if it helps, then great!

 

 

 

If you use a generic tyre/wheel size calculator from the interweb, there's apparently very little in it between those two tyre sizes.

Would be interesting to plot OBD2 vs speedo vs GPS and see what ones look "out".

 

J.

i wouldnt trust your sat nav to give an accurate speed reading  

I read somewhere recently that sat nav speed readings are 100% accurate.

I read somewhere recently that sat nav speed readings are 100% accurate.

 

They are definitely quite accurate, I have often drove behind lorries which have a sign stating that they are limited to 56mph - my speedo reads 60mph but sat-nav reads 56mph if I stay at a constant speed/distance

If you use a generic tyre/wheel size calculator from the interweb, there's apparently very little in it between those two tyre sizes.

Would be interesting to plot OBD2 vs speedo vs GPS and see what ones look "out".

 

J.

 

Interesting.   Having had a look, I can see:

Rear tyres: 578mm diameter

Front tyres: 583mm diameter.  

0.87% difference.  Maybe the tyres aren't the cause and the speedo is just very inaccurate at circa 30mph.  

Can't say I noticed. Wheel bearings have been changed as well as abs sensors on the front, some genuine parts and some not. Might be the cause if the pickup is slightly different?

Have you checked live data to see if all four wheels pickup the same speed?

Why not swap the wheels front to rear and see what the Speedo says then ?

Can't say I noticed. Wheel bearings have been changed as well as abs sensors on the front, some genuine parts and some not. Might be the cause if the pickup is slightly different?

Have you checked live data to see if all four wheels pickup the same speed?

 

Good idea! I'll stick VCDS lite on it it has the capability? I'm starting to think it's just a Fabia quirk.  It seems to be mostly out at 30 with a more expected tolerance at higher speed.

 

Will also try swapping wheels when I next meet up.

 

Thanks again for everyones input 

If you have an Android phone, an ELM327 OBD2 interface is under £7 on eBay. And Torque Lite (app) is free on Google play store.

 

Even if phone doesn't have GPS, compare satnav (GPS) vs OBD2 speed (from ECU/car)  vs speedo. I'd guess that the inaccuracy is in the speedo itself.

 

Incidentally, when I did the above, GPS read highest with OBD2 within 2% with the speedo about 6-7% down. Though that was at motorway speeds.

My 1.9 TDI pd100 was bought with 195 60 14s fitted all round

Interesting. Having had a look, I can see:

Rear tyres: 578mm diameter

Front tyres: 583mm diameter.

0.87% difference. Maybe the tyres aren't the cause and the speedo is just very inaccurate at circa 30mph.

Wear on the tyres has an effect. If the tyres have 10mm tread when new and 2mm when done, that's a difference of 16mm (roughly 2.75%). The error is worse when the tyres are at minimum tread. By law, the speedo can never read low.

You're actually coming up against a known issue.

 

VW acknowledged a problem back in 2007 and released TSB 90 07 03 for this very reason.

 

All cars post 2007 were instructed to be re-coded to address the issue.

 

http://tech.bentleypublishers.com/servlet/JiveServlet/download/109-25248-335279-5941/vw.tb.90-07-03.A5speedo.pdf refers.

 

As an aside, you can remedy this using VCDS if you have a copy, or know a friendly person with a copy to do it for you.

Edited 18 May, 201412 yr by Chris Coates

Can this be somehow with vcds? or anything else?

 

I need very badly calibration for -03 skoda 1.2, because meter shows ~10% under real.