[jalsa777]

Hello people,
A really simple question here:
With the multiple electronic safety features in modern cars, would a tire upsize (slight increase in diameter) affect the way the car functions? More specifically, would it have a negative effect on things like the ESP, ABS, TPMS, etc.

Case in point here is that a friend has an Audi Q7 with 255/55 R18 tyres. He has to replace them and 255/60 R18 are coming out to be much cheaper (don't ask me how). He wants to know if he should go with the larger ones or keep the original size? The Diameter changes by around 13mm with this upsize.
What would you suggest?

[Jeroen]

Originally Posted by jalsa777

Hello people,
A really simple question here:
With the multiple electronic safety features in modern cars, would a tire upsize (slight increase in diameter) affect the way the car functions? More specifically, would it have a negative effect on things like the ESP, ABS, TPMS, etc.

Case in point here is that a friend has an Audi Q7 with 255/55 R18 tyres. He has to replace them and 255/60 R18 are coming out to be much cheaper (don't ask me how). He wants to know if he should go with the larger ones or keep the original size? The Diameter changes by around 13mm with this upsize.
What would you suggest?

Doesn't make a blind bit of difference for the (safety) electronics.

The only thing it will do is affect is his speed, miles driven and fuel consumption as the latter is a function of speed/distance. Different tyres will give you a different circumference, so a different reading on these times.

I've never understood this quest for bigger tyres, but if it makes him happy or feel better about his wallet go ahead by all means

Jeroen

[ajaypjayaraj]

Thumb rule is to keep the difference between circumference of the stock tyre and upsize tyre within 2%. It is not a rule but some guidance to follow to minimise the adverse effects due to change in tyre size.

There are many calculators online and in app stores you could use to check that out. Here is one such online calculator - http://tyrestore.in/upsizing.htm

The tyre size you mentioned is more than 2% change. It shows 3.46% change which is not really advisable.

The cost of tyre is less when the profile is higher and vice versa. The lower profile tyres have stronger side walls which could be translating to their higher cost.

Regarding electronics, theoretically the change in circumference does have impact on it as the stock circumference is fed into the module. Even tyre pressure matters if we are going by theory. So when you alter the wheel dia. without changing the parameters fed into the car electronics, obviously there would be a slight error in calculation done by the car electronics. But in real world, the difference could be just marginal enough to be noticed. It wouldn't be that much of a concern with minor changes. Actually the thumb rule of 2% could be to minimise all these effects. So keep that in mind and hunt for a tyre size and your friend should be fine. Even if he is to upgrade to the tyre size you mentioned, if it does not scrap the inner walls of the wheel well, most drivers wouldn't notice the other changes. Better to keep it within 2% difference and be safe.

[dhanushs]

@OP, I think, the difference (in this case) isnt significant enough to effect the electronics, like Jeroen has posted.

Originally Posted by Jeroen

I've never understood this quest for bigger tyres, but if it makes him happy or feel better about his wallet go ahead by all means

Wider tyres (for vehicles with puny stock tyres) for better grip, and taller tyres for better GC.

[Mr.Boss]

ABS works by calculating / comparing the relative RPMs of each wheel. When all 4 wheels are of same diameter (upgraded / downgraded) there isn't much impact on them. To be simple tire upgrade has got nothing much to impact the electronics parts (except the speedo / odo) but it does affect mechanically (braking effort / efficiency, speed, higher input load to mechanical parts, etc)

[karan561]

I am running 245/40 R18 vs 225/45 R18 (stock Size) on my BMW. Electronics haven't been hampered & if they would be showing different reading in form of speedo error or different odo readings its all negligible.

In fact the TPMS in my car is designed for Runflats, but my upgraded tyres are tubeless & still the TPMS hasn't been able to figure out that change & is working perfectly fine

Other things like ABS, ESP will probably work / perform better, simply because however modern the car may be they still want to achieve good FE nos. (at least the regular sedan / hatches) & the OE tyres are mostly undersized, so there is always a scope for improvement with upgraded tyres.

So i believe your friend should go ahead with the higher profile tyres for his Q7.

The Q7 can easily swallow this upsize

[ajaypjayaraj]

Originally Posted by Mr.Boss

ABS works by calculating / comparing the relative RPMs of each wheel. When all 4 wheels are of same diameter (upgraded / downgraded) there isn't much impact on them. To be simple tire upgrade has got nothing much to impact the electronics parts (except the speedo / odo) but it does affect mechanically (braking effort / efficiency, speed, higher input load to mechanical parts, etc)

In the case of OP, it must be fine. But in some cases like that of an AWD electronic differential like Haldex, it would matter especially at higher speeds on slippery surfaces, by detecting the slip a little more delayed or quicker depending on which way the change was made. Might not be humanly detectable. But it does effect in such cases. How much is the change is math. Q7 quattro uses torsen differential which is mechanical, so that shouldn't matter.

Originally Posted by karan561

I am running 245/40 R18 vs 225/45 R18 (stock Size) on my BMW. Electronics haven't been hampered & if they would be showing different reading in form of speedo error or different odo readings its all negligible.

In fact the TPMS in my car is designed for Runflats, but my upgraded tyres are tubeless & still the TPMS hasn't been able to figure out that change & is working perfectly fine

Other things like ABS, ESP will probably work / perform better, simply because however modern the car may be they still want to achieve good FE nos. (at least the regular sedan / hatches) & the OE tyres are mostly undersized, so there is always a scope for improvement with upgraded tyres.

So i believe your friend should go ahead with the higher profile tyres for his Q7.

The Q7 can easily swallow this upsize

Karan, you took care of the outer Dia. with the profile change, so why would there be any considerable change enough to be detectable. That is how I upsize tyres too. I have upgraded to various size in the range of 16-19" rim size and 215 to 255mm tyre width on the same car keeping the outer dia. almost same (circumference difference may be within 1%) by choosing the correct profile.

Sorry I din't get what you meant by TPMS is still working fine? What change were you expecting when you upsized keeping the outer dia. almost same and replaced runflats with tubeless? OT, does F30 come with ABS based TPMS?

Other things might work to advantage or disadvantage depending on what driver assisting or safety aid are we talking about.

@ OP, sorry for using your thread. I am not against tyre upsizing. Just that it is an interesting topic which I love to discuss - car electronics. So just pooling in information for discussing about tyre size effect on car electronics. Q7 would probably swallow this minor change without showing any detectable change in driving dynamics. I am just discussing points beyond just your case. Thanks.

[Akshay1234]

Originally Posted by karan561

In fact the TPMS in my car is designed for Runflats, but my upgraded tyres are tubeless & still the TPMS hasn't been able to figure out that change & is working perfectly fine

The TPMS bases itself on whatever is the tyre size when you reset, so technically even if you don't stick to the correct upsize it will work correctly. BTW the TPMS in the F30 will work better with tubeless, since in case of a puncture the difference in rotational speed of the tyre will be more easily noticed than with a runflat, with its super stiff sidewalls.

Originally Posted by ajaypjayaraj

OT, does F30 come with ABS based TPMS?

Yes it does. Most BMWs in India do.

[GTO]

It wouldn't.

Manufacturers themselves sell the same car with varying tyre sizes in different variants (or markets). Higher variants - with essentially the same electronics - get bigger wheels / tyres in so many cars!

[ajaypjayaraj]

Originally Posted by karan561

Do go through these two links;

1) http://blog.tirerack.com/blog/docs-a...essure-sensors

2) http://www.autec-wheels.de/tpms.html

^ these are self explanatory & shall explain why i said " Both "

Just went through some bimmer forums for the answer meanwhile. Now this passive TPMS which we are talking about, which uses ABS sensors, are referred to as FTM since it doesn't really check any individual tyre pressures but only keep comparing the difference in wheel rotation speeds of all 4 wheels which would change due to the deformation of a deflated tyre and trigger an alarm if any change is detected. Since it only does that, its called a FTM (Flat Tyre Monitor).

The other one is TPM (Tyre Pressure Monitor). The sensor inside the wheel is for that. I should have checked when I took the car to BMW for the torn tyre replacement if it had sensor inside the wheel. This sensor comes with a built in life time battery (as per some bimmer forums) and communicates with car by radio signal. If your car has a sensor within the wheel, it has to be TPM.

Either the above or BMW uses a transponder within the wheel for more accurate reading of wheel rotation by the ABS sensor placed outside. I can't think of any other reason for why that sensor is there within the wheel if the car is using passive TPMS or FTM only.

Originally Posted by GTO

It wouldn't.

Manufacturers themselves sell the same car with varying tyre sizes in different variants (or markets). Higher variants - with essentially the same electronics - get bigger wheels / tyres in so many cars!

But when they increase the wheel (rim) size, they go for lower profile tyres usually keeping the outer dia. within say 2-4% depending on the tolerance of the electronics they use. Is that not the case?

[Akshay1234]

Originally Posted by ajaypjayaraj

That is no news. You said there was some sensor in the wheel. What I asked is, Is that a tyre pressure sensor you meant which is inside the wheel?

ABS based is passive TPMS. But that does not require any sensors within the wheel apart from the ABS sensor which is placed outside the wheel AFAIK.

In India in BMWs we get flat tire monitoring system (TPLS), and not the proper tire pressure monitoring system(TPMS). I think its only the CBUs which get the proper system. Infact when resetting the system it even says Tire Pressure Loss Warning reset. Also if you notice it never gives you the exact tyre where the pressure loss has occurred, which I think does happen with TPMS.

Originally Posted by ajaypjayaraj

But when they increase the wheel (rim) size, they go for lower profile tyres usually keeping the outer dia. within say 2-4% depending on the tolerance of the electronics they use. Is that not the case?

Again from what I've seen its always within a certain threshold, I don't think they deviate more than 2%, but the speedometer is always calibrated correctly for the variant.

[rpunwani]

Originally Posted by jalsa777

Hello people,
A really simple question here:
With the multiple electronic safety features in modern cars, would a tire upsize (slight increase in diameter) affect the way the car functions? More specifically, would it have a negative effect on things like the ESP, ABS, TPMS, etc.

Case in point here is that a friend has an Audi Q7 with 255/55 R18 tyres. He has to replace them and 255/60 R18 are coming out to be much cheaper (don't ask me how). He wants to know if he should go with the larger ones or keep the original size? The Diameter changes by around 13mm with this upsize.
What would you suggest?

Apart from what our fellow members have advised, I suggest that you ask your friend to please check the Speed & Load Rating on the original tyres as well as that on the tyres he intends to buy. This has a significant impact on the price of the tyre.

Case in point: The OEM Pirelli 215/60 R16 W97 tyres on the Passat are about Rs. 16.5K. You can get lower rated tyres i.e. W93 for Rs. 11k. Almost 35% cheaper !

[Rev_That_V8]

Originally Posted by GTO

It wouldn't.

Manufacturers themselves sell the same car with varying tyre sizes in different variants (or markets). Higher variants - with essentially the same electronics - get bigger wheels / tyres in so many cars!

if the upgrade is tiny/big it still will affect in the long run as the error though tiny, is cumulative.
To combat this, Ecu's of different market vehicles are usually configured by manufacturers differently (meaning - internal data values in the register of the ECU are written to match the correct market tire for Odometer readings and etc.).

-- Technical info --

While actual distance traveled by the vehicle is completely dependent on the circumference of the tires, the vehicle speed can be taken by connecting a tacho to the transmission whose drive-shaft diameter is compared with the actual wheel diameter and then arrived at the actual speed of the vehicle.

The parameters such as vehicle speed, Odometer value , DTE(distance to empty ) , FE will be wrongly displayed as all these have Distance in their calculation.
For Odo reading, circumference of the tire is multiplied to the number of times the drive-shaft rotates to make the tire move 360 degrees to arrive at a number which is translated into distance which will be wrong as the new circumference is different.

ABS is another control system which takes vehicle speed (to translate into traction) into calculation. it will get affected as well, no matter if you go wider/larger in tires.

Airbag deploys in a crash (depends on design of the manufacturer to include vehicle speed) with other inputs.
So, that gets affected as well.


If your vehicle has any other ECU getting this parameter indirectly (like a "service due info" on cluster which depends on distance you traveled post last service) it will show an earlier/late indication as well .

cheers ,

[MadTiger]

Hey Guys,

Just yesterday I was thinking about this and there you go, here's the thread! I have upgraded my stock tire from Bridgestone 165/80R14 to Yoko 'A' drive 185/70R14. As per tire upgrade website data my upgrade has caused difference in circumference by -0.81%. The top end comes with the same tire spec, but I would be interested in re calibrating my speedo, is that a possibility?

[ACM]

Manufacturers provide bigger tyre sizes in higher variants. The net difference in diameter is less than 2% Though up to 3% is permissible as per some sites.

In my Laura I had an almost 4% upgrade 205/55 changing to 225/55. There were no electronic errors. I had a more comfortable ride and the vehicle stopped hitting speed bumps.