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

The numbers on the tyres sidewalls do not always give the same result. Once you measure tyre circumferences with a tape you can easily check that.

Different Brands /Tyre Treads, then obviously once on rims of different widths the pressures you inflate to.

 

One of my friends recently bought a secondhand Citroen C5 automatic (well somebody has too!). He complained that the auto box was playing up and would not sometimes change gear when it should. He took it to his tame mechanic who diagnosed that all tyres were a different type and although all the same diameter, width and aspect raio the circumferences were all different (as Offski said). Once he fitted 4 tyres of the same type all the automatic gearbox problems dissapeared.

8 hours ago, Wino said:

The odometer error will be the same as the speedo error.

 

 

It would be quite easy for the car to measure the correct speed (or there abouts) for the odometers sake and have the adjusted speed (+3/4mph) indicated on the speedo.

 

I don't know if this is actually how it works but I can't see car manufacturers letting all those extra miles clock up unnecessarily. 

 

 

My experience with a variety of vehicles over the last 10 years or so is that the odometer accuracy is usually withing a couple of % or actually spot on (one exception being a Hyundai Sant Fe mk1 that was at least 5% optimistic) but the speedo is invariably between 5 and 10% optimistic, with the aforementioned Hyundai registering 12% optimistic.

 

The digital speedo display on the Octavia always reflects the analogue speedo so is pretty useless really. I use GPs speeds to minimise travel time or speedometer speeds to maximise economy.

A review of an expensive  W12 engine Bentley said that its odometer was 4% optimistic so our 15 yo Toyota 1.3L Yaris, with centrally mounted digital speedo which is surprisingly is only 3% optimistic is theoretically the faster car

 

7 hours ago, SuperbTWM said:

 

It would be quite easy for the car to measure the correct speed (or there abouts) for the odometers sake and have the adjusted speed (+3/4mph) indicated on the speedo.

 

I don't know if this is actually how it works but I can't see car manufacturers letting all those extra miles clock up unnecessarily. 

 

 

Good point.

9 hours ago, SuperbTWM said:

 

It would be quite easy for the car to measure the correct speed (or there abouts) for the odometers sake and have the adjusted speed (+3/4mph) indicated on the speedo.

 

I don't know if this is actually how it works but I can't see car manufacturers letting all those extra miles clock up unnecessarily. 

 

 

 

On the mk1 Octavia with climate control, if you pressed the right combination of buttons, the speed would be displayed on the climate control screen. The speed shown was *slightly* above the speed shown on the GPS, and about 6% lower than the speed shown on the speedometer.

 

The Octavia behaves as you suggested.

 

I've not checked the Citigo though.

22 hours ago, C50 said:

 

I have explained why I am appalled and only here to see if there's a solution. We are in the year 2018 where nano tech exists! All my previous non VAG pre 2004 vehicles were never this out (Japanese, Proton, Kia).

 

I guess I like things to be accurate and especially on a speedo if I look at it daily. They ought to sack whoever is calibrating the speedos at the factory!

 

 

 

 

There is an internationally agreed specification for the accuracy of a speedometer which the manufacturer is required to adhere to. The speedometer system on the Citigo operates to this specification and is designed to operate within this specification for the lifetime of the vehicle. No need to sack the speedo person.

 

I'll happily agree that the speed indicated by a GPS is likely to be more accurate but it will not be as reliable. which is fair enough as there are no regulations that require it to be.

 

I'm not sure where the nano tech comes in.