Tested using cruise control, I noticed that my speedo was reading 10% down on my relatively new to me octavia with new 225/45 R17 tyres.

VCDS -> Modules -> Instruments -> Long Coding

Mine was originally set to option 0, option 1 made no difference, option 2 has made the speedo very accurate, I estimate reading 0.5mph lower than GPS. There are a total of 8 options.

Since the change naturally the MPG has increased by 10% as the car is accurately recording the mileage it travels. However, I have to assume that my 126k is in fact 139.5k.

Used info from this thread to assist:

https://forums.ross-tech.com/index.php?threads/402/

Edited 9 September, 20259 Sep by e301988325i
More info

16 hours ago, e301988325i said:

.....Since the change naturally the MPG has increased by 10% as the car is accurately recording the mileage it travels. However, I have to assume that my 126k is in fact 139.5k.

Used info from this thread to assist:

https://forums.ross-tech.com/index.php?threads/402/

I always reset the trip computer and manually check my MPG at every fill up

Divide mileage by litres, times result by 4.546 (No. of litres per gallon (UK)).

This shows you the actual MPG. 👍

Computers? We don. need no steenkin' computers ....

On 10/09/2025 at 10:35, Sofo said:

I always reset the trip computer and manually check my MPG at every fill up

Divide mileage by litres, times result by 4.546 (No. of litres per gallon (UK)).

This shows you the actual MPG. 👍

Computers? We don. need no steenkin' computers ....

It doesn't give you accurate result if the car isn't recording all of the miles it's driven!!!

The odometer for miles covered is probably very accurate.

The same source data will be used for both, but the speedo artificially calibrated to over-read, as required by law in some countries such as the UK.

Edited 8 October, 20258 Oct by Breezy_Pete

3 hours ago, Breezy_Pete said:

The odometer for miles covered is probably very accurate.

The same source data will be used for both, but the speedo artificially calibrated to over-read, as required by law in some countries such as the UK.

Thanks Pete - do you have a source for that?

A pity that roadside measured mile markers no longer appear to be a thing.

Measure it on a long journey and compare with what google maps says. Best if it's a straight line single road trip probably.

Someone clever on here pointed out to me that just because the same data (from ABS module) is used for speed and distance, they don't both have to be fudged in the same way.

Edited 8 October, 20258 Oct by Breezy_Pete

Since I have had access to a gps I have confirmed that nearly all my cars odometers are accurate (depending on tyre wear) within a couple of percent. The exception was a Hyundai Santa Fe mk1 V6 which turned out to be about 8% optimistic.

The speedometers on a previous mk2 and current mk3 Octavia were/are about 7% optimistic. The Santa Fe was 10% optimistic.

I don't really care about speedo accuracy once I know the variance as I can allow for it. An optimistic speedo is useful if lending the car to my son :)

Fuel economy display figures are based on fuel used and distance travelled and do not use the display speed. So I always do a quick calculation when I refuel with actual distance and refuel amount.

My 2014 Octavia mk3 1.4tsi fuel display was slightly pessimistic when new but over its life and 117k km the display has progressed through very accurate to increasingly optimistic, so I now have to take off 0.3l/100 off the display to a more accurate figure.

A simple test is to set the cruise control (which is related to the speedo display) for a set figure (say 60kph) and then reset the average speed display for journey while in motion, after a few hundred uninterrupted metres the average speed calculated by distance travelled is shown and can be compared against the speedo.

I would have hoped that VCDS would have allowed adjusting the displays for speed, distance and consumption separately, but according to the OP this might not be the case?

Edited 8 October, 20258 Oct by Gerrycan
Grammar

Speedos by law in Australia and other countries as well MUST read more than actual speed and NOT less than actual speed. Has been law in AUS since July 2006. My Tiguan was 7 fast at 110 and by altering the wheel circumference with OBDELEVEN I got it to almost bang on. Dont want it correct on new tyres because it will be illegal when they wear down. ODO is not affaected they reckon but Im not sure on that as would think they both use the same ABS sensor to calculate distance and speed. The speed is calculated by revolutions per kilometer so would think the OD is done the same

The ADR Rule in AUS say they must read more than actual and can be up to 10 + -4 at 100kph so could read 114 and be legal. Strang but true

All my modern cars (from 1979 onwards) speedos have read fast by up to 10%; strangely my 1937 Morris 8 is spot on at 40mph!