In an typical Octavia, where is the tyre size (rolling diameter) coded into the ECU and how difficult is it to change?

I'm essentially looking for calibration for different wheels/tyres. At the moment the ECU knows the exact speed I travel at (scangauge matches GPS) but the speedo reads the standard 5% under.

Just fit some 245/45 R 18 (Wrangler AT/R if you can find them) and your speedo will be spot on. It'll give you another 10mm ride height/wading depth great mud and snow traction and looks gnarly too.

...and if you don't want to change wheels, some 225/55s (Scorpion STRs perhaps) will do the same.

Edited 28 September, 201213 yr by Jeeves

Don't tempt me.

But I'm actually wondering about speedo calibration for the other direction. 5% I can live with, but I have a standing offer of 4% smaller tyres again if/when I need them and 9% is a bit much. If it were possible to crank it through VCDS or similar it'd be excellent.

Thanks Mike. Do you know which code Octavias with 205/55R16's run in that box?

I will be ordering VCDS shortly.

The reason for asking, my scout has 225/50R17's and I have an offer of free 205/55R16 snow tyres if/when needed. I didn't realise you could change these readings on the fly, that takes out a lot of the need for prior planning.

I take it you don't have maxidot, as there's a winter tyre setting in there......

Is the winter tyre setting different from on a Fabia.

Can you adjust so that it knows you have Winter Tyres on of a different rolling radius ?

On a Fabia,

You set it to 'bong' when you exceed the speed that you set it to warn you that you have 'Winter' tyres on.

(on a Fabia, changing from 205/40/17 - 215/40/17 actually makes everything, milometer and speedo read dead on,

well tyre tread and type dependant)

Remember to re-set your 'TPMS' when you change tyre sizes.

It reads the tyre size and changes, not actually the pressure.

http://www.kouki.co....size-calculator

http://www.michelin....ld-weather-tips

A handy explanation on setting tyre pressures in the cold

george

What a waste of time that is. So if I set my speed waning to 70, then it will bong at that point. I can use the apparently misleadingly named 'winter tyres' setting to put that +/- 5mph but I could have done tat anyway simply by changing the speed warning to 65........

Am I missing something or is the winter tyre setting completely pointless?

It is excellent IMO.

Set it to 65 mph, 70 mph or what ever.

You know you have Winter or Snow tyres fitted,

it lets the other driver, SWMBO or Servicing mechanic or who ever know to not exceed a safe speed for the tyres fitted, possibly on a nice day where they might push on a little.

It leaves you free to set your other 'speed warning to what ever you want that at '40mph maybe' !!

Or set your speed limit warning to 70 mph & then you can just use the Winter Tyre one as a higher speed warning.

Double trouble warnings.

http://www.btmauk.com/data/files/Replacing_Car_Tyres_1_June_2011.pdf

george

Remember to re-set your 'TPMS' when you change tyre sizes.

It reads the tyre size and changes, not actually the pressure.

You do not need to re-set the TPMS as it detects the 'puncture' as one wheel turning faster than the other 3 (a tyre loosing air will have a reducing rolling circumference)

This does not alter when you fit a full set of tyres with the same rolling circumference as each other.

Am I missing something or is the winter tyre setting completely pointless?

The winter tyre setting is to be used when you fit lower speed rated tyres on, so you do not forget you have exceeded the recommended speed rating of the tyres fitted.

re TPMS.

Anyone can obviously ignore the 'Owners Manual' if they wish.

'Just do nothing'

It advises you in the 'Owners Manual' to 'Re-Set' the TPMS after adjusting your tyre pressures or changing tyre size.

'What do they know?'

** Cant see the problem myself with pushing a button and resetting each time you have checked or readjusted you pressures in cold weather or such.**

http://www.michelin.co.uk/tyres/learn-share/care-guide/cold-weather-tips

'Gizmo69's

advice is probably best to adhere to. He has your best safety at heart!!!

Maybe best to understand your Speed Ratings.

http://www.tesco-tyr...lp/speed-rating

http://www.btmauk.co...1_June_2011.pdf

george

The only time my TPMS has set off a warning (so it has detected a difference in rolling circumference) is when I picked up a slow puncture (screw in the tread).

My winter tyres have been on the car twice, so in total the wheels and tyres have been swapped over 4 times, yet the TPMS is perfectly happy with this.

Now if you were only fitting a pair of new tyres then I would agree, you need to re-set the TPMS as you will have different rolling circumference tyres on the car.

When all 4 are swapped over you are not altering anything.

What you are doing is checking that it is set and you also have some idea if the system is working or not.

Too many now never check tyre pressures or have no idea how or why a system is there.

Like i said, its a 'Press of a finger' to check.

That is your System 're-set' total cost nothing,

exerted energy, less than picking your nose.

It is always interesting to hear others ideas to 'safety' and basic checks.

Why even check you oil these days,? a light comes on when it is too low.

Its why i would never trust a Garage or Tyre fitting place to do things.

Check yourself always, & sometimes do the safe thing, do not listen to others.

That includes me,

but more so, those that think doing nothing is the right way.

george

Just been in the car so had a play, so had already now seen that its effectively a 'second' speed warning system.

Not sure I see the point TBH but that's probably because my driving is so mixed that the last thing I'd want would be a speed warning that came on low - I keep the main one at 95 which keeps me out of licence losing territory if I get too enthusiastic on the m25

The less 'bongs' the better for me!

I know when I'm going too fast by checking the speed at which the scenery passes, or even better the speedo

I had a play with this today. Setting the last digit from 2 to 1 has dropped my 100km/h error from the standard 4-5km/h to around 1-2km/h. I'm perfectly happy with that.

I've also cranked the fuel consumption display by 6%. That being the error my scangauge logged over about 4 tanks.

Shame I can't hack around with more really. Go-on VAG, let us play with AC evaporator temps, haldex settings and the like.

Don't forget when re-calibrating your speedo that the difference in rolling diameter between brand new and fully worn tyres is ~1.5%

About 80% of the time a GPS is in the car anyway. This puts the error at the same as the rest of the fleet.