Hello all,

I believe the Karoq has a speed sensor on each wheel. Does anyone know how much difference there can be between the recorded speed on each wheel before the ABS system throws a fault?

Is it 2%, 3%, 4%, ?% difference in rolling circumference (ie. wheels with different rim+tyre size). How much can be tolerated Front to Rear, and Side to Side?

As a second question, where does the speedometer take its reading from?

Thanks in advance.

Sorry no idea. Are you asking because of the TPMS. Tyre pressure loss , difference in setting and tyre circumference / total diameter.? You can put different pairs by size on front and back and set the TPMS and not get warnings. Within sensible differences.

I think the speedometer is probably driven from the gearbox output shaft.

No, not asking because of the TPMS. Although a (partially) deflated tyre would also change rolling circumference and wheel revolutions per second compared to the other wheels, I believe the TPMS compares tyre pressures directly from a wireless thingy attached to the back of the tyre valve inside the tyre.

I am more interested in how much variation in rim+tyre (ie. wheel circumference) variation between wheels on a car running straight on a level road can be tolerated by the ABS/ESP system before it indicates a fault. For example, a 225/55 R18 wheel/tyre would have a theoretical rolling circumference of 2214mm and a 235/40 R19 would have a circumference of 2107mm. The first wheel/tyre would therefore be spinning about 5% less to cover the same distance as the second. If one of these was fitted to one (or two) of the wheel positions, the wheel speed sensor would detect this.

At what point (at what percentage difference) front-back or side to side would the ECU dislike this and throw a fault?

This affects what alternative wheel/tyre combinations I might be able to use on the car (quite apart from brake & wheel arch considerations).

2 hours ago, thamestrader said:

I think the speedometer is probably driven from the gearbox output shaft.

Are normally a sensor in the ABS system in a back wheel and work on revolutions per kilometer.. Mine was reading quite over ( as it can legally) and I adjusted it with an OBDELEVEN to be more accurate

13 hours ago, warmrain said:

No, not asking because of the TPMS. Although a (partially) deflated tyre would also change rolling circumference and wheel revolutions per second compared to the other wheels, I believe the TPMS compares tyre pressures directly from a wireless thingy attached to the back of the tyre valve inside the tyre.

I am more interested in how much variation in rim+tyre (ie. wheel circumference) variation between wheels on a car running straight on a level road can be tolerated by the ABS/ESP system before it indicates a fault. For example, a 225/55 R18 wheel/tyre would have a theoretical rolling circumference of 2214mm and a 235/40 R19 would have a circumference of 2107mm. The first wheel/tyre would therefore be spinning about 5% less to cover the same distance as the second. If one of these was fitted to one (or two) of the wheel positions, the wheel speed sensor would detect this.

At what point (at what percentage difference) front-back or side to side would the ECU dislike this and throw a fault?

This affects what alternative wheel/tyre combinations I might be able to use on the car (quite apart from brake & wheel arch considerations).

Wireless (and expensive) on some cars but VW group vehicles mainly use the ABS exciter built into the hubs..........

@Berisford By ABS exciter I presume you mean ABS exciter ring which is part of the Wheel rotation speed sensor mechanism? I don't understand how that could measure tyre pressure (TPMS), especially when a tyre pressure warning can activate even when the vehicle is stationary or hasn't moved at all?

2 hours ago, warmrain said:

@Berisford By ABS exciter I presume you mean ABS exciter ring which is part of the Wheel rotation speed sensor mechanism? I don't understand how that could measure tyre pressure (TPMS), especially when a tyre pressure warning can activate even when the vehicle is stationary or hasn't moved at all?

Has your vehicle been retro fitted with a wireless type pressure monitor system?

@warmrain Could you just say what it is that you want to do to your Karoq that many others might not already have done?

I am sorry if I am causing any confusion.

Primarily I would like to understand at a more fundamental level how the ABS/ESP system integrates wheel rotation speed(s) and what the tolerances are. This would help in making a more informed decision.

After buying my Karoq recently through a Skoda main dealer I have discovered that they fitted two new tyres to the rear wheels but of a different size to the existing fronts. I am pondering whether (a) I should go back and make a fuss about the mismatched tyres, and (b) at the very least -- a situation can arise whereby if I have a puncture and need to put on the spare wheel which is of yet a different rim/tyre size with different rolling radius/circumference, what will happen to my ABS system when it is faced with three different measurements of road speed?

There's only a couple of percent difference between the circumference of the specified tyres on 17, 18 & 19" rims.

My understanding of the TPMS is that when you set it, that's the RPM it uses to measure deviance from. It's possible to lose a little pressure, reset the TPMS & it will happily carry on using the new value as the default. So I suspect you could in theory have slightly different circumferences on all four wheels & it wouldn't care.

Although I wouldn't be happy with running mismatched sizes, simply because your insurance company could regard the wrong tyres as an undeclared modification in the event of a claim. So what sizes have you ended up with?

An initiallisation causes the system to recalibrate itself as you drive. Those roatation speeds presumably go into a memery somewhere and the syastem then tracks the rotation speeds. It will take some time for a pressure loss to be noted as it needs to ensure it is not just a difference in speed between the inner and outer wheel on a corner. The system cannot respond quickly and a catastrophic tyre failure will not cause an alarm.

These syatems usually respond when the pressure drops by ~7psi. I have no idea what the difference in rolling radius will be if you lose this pressure, and this could be different depending on how the car is loaded. The Skoda system is different from our other car as it actually indicated which tyre is low; on the other one you actually have to figure it for yourself.

@warmrain Thanks.

Do not be shy. What size have the dealer fitted and what size are the ones not changed.

As it is a spare might be slightly different to both the front and rear. You just reset the TPMS which has no Valve Sensors in the wheel.

So do you have a FWD Karoq

or one with Part Time all wheel drive. (Haldex)