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Getting a bit sick of the P7 Cintaruto tyres. Very noisy on coarse road, thinking of swapping it out for a Michelin Super Sport.

The tyre size (235/40/R19) that came with the Superb's 19" rims is pretty expensive here in Australia (Around $400AUD each). 

However, the 245/40/R19 size tyres are nearly half the price ($230-$250 AUD each) since they're much more common here due to them being used by other manufacturers' large cars. 

Just seeing if anyone have tried using the 245/40/R19 tyres instead of the OEM size (235/40/R19) with good results. Just seeing if it will affect handling / ride quality. Also see if there'll be rubbing on wheel arches?

Unfortunately, in Australia you can't try out the tyres before you buy. So I will need to purely rely on other people's experiences. 

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4 hours ago, cyberkank said:

Unfortunately, in Australia you can't try out the tyres before you buy.

You mean there are countries where that's possible? ;)

 

I seriously doubt the extra 10mm will cause any physical problems but what about insurance ones?

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Using the link provided by MartiniB I have entered the tyre sizes, the comparison shows that your Speedometer will under read i.e when it shows 80kph (50 mph) you will actually be doing 50.6 mph. 

 

Capture.PNG

Edited by Prykey
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3 hours ago, Prykey said:

the comparison shows that your Speedometer will under read i.e when it shows 80kph (50 mph) you will actually be doing 50.6 mph. 

exactly the opposite,
with bigger tires car's speedometer shows now closer to reality -
+2km(90km/h)
with stock were +~5km, when compared to GPS

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9 hours ago, MartiniB said:

exactly the opposite,
with bigger tires car's speedometer shows now closer to reality -
+2km(90km/h)
with stock were +~5km, when compared to GPS

 

 

?? The speedo reading doesn't change.:thinking:

 

Car speedo tells you X miles an hour (whether that is exact is by the by, comparison with GPS is another discussion entirely, not least that it's an average and not real time).  

 

Where do you get +2 KMH @90, the change for the example above is +1.1%?

 

Change for a bigger tyre and the car will still tell you X miles an hour but with the rolling circumference having increased 1.1% increase will be X+0.6 @50 mph and x+1 @90 mph, the speedo will therefore under read.  

 

It might have been reading on the nail or close, i.e. X=50 mph, before but will not accurately reflect the changed distance per wheel rotation. 

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7 hours ago, Bud said:

Car speedo tells you X miles an hour

Car's speedometer is dependent on wheel's Revs/Mile while GPS doesn't care what size tires are installed

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8 hours ago, MartiniB said:

Car's speedometer is dependent on wheel's Revs/Mile while GPS doesn't care what size tires are installed


What is the real speed and why?

 

Car's speedo calculates speed based up wheel size, so if you change the tyres/wheels but cannot enter a new circumference you have introduced error.  

 

If the diameter of the wheel/tyre alters, the extrapolation calculation will be incorrect. For example, the diameter will increase if you put new tyres on the car (more tread, which wears down over thousands of miles) or increase the tyre pressure.  This means that, for each revolution of the wheel, the car is travelling further, meaning your speed is greater.

 

If the diameter decreases (eg – worn tyres, less air in the tyres, a different brand of tyre with slightly different dimensions, more load in the car weighing it down and compressing the tyres), then the car will be travelling a shorter distance for each revolution of the wheel, therefore you will be going slower.

 

The system is dumb and does not know weather you have done 3m or 3.2 m per revolution

 

Unless, of course, we can we recalibrate the wheel circumference. 

 

GPS is an averaging function based upon distance travelled over time but if you have a large error possibly due to multipath or as you seem to complain about not getting a signal through your car windscreen, the distance between sample points and therefore the average will be inaccurate.  Yes it has no way of knowing if you have skateboard wheels or fire engine wheels but it also only tells you what it can calculate and this will include errors, always.

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@cyberkank The extra 10mm tyre tread width will probably make little difference to handling, you may notice fractionally stiffer steering and increased tyre wear. It certainly won't improve nor diminish braking efficiency.

 

The reduced circumference of the tyre will indeed cause issues with the speedometer (as already mentioned), odometer, and possibly the ABS based tyre pressure monitor.

The speedometer will read less than actual speed, by about 1 mph at 70 mph.

The odometer will record a greater distance travelled and that difference will only increase over time and distance.

As for the ABS based tyre pressure monitoring, bear in mind how often it is already prone to causing errors on stock OEM tyres!

 

I can only comment on the UK when I mention that, by EU directive for Type Approval and also the Construction & Use Regulations as amended, a speedometer may record 10%+6.25 mph higher than true speed but must not record less than true speed. That being said, the speedometers of cars in the UK are never routinely tested for accuracy. The exception being those fitted in police traffic cars for speed enforcement purposes, or if you are involved in a fatal or life threatening/altering collision in which case your car will be examined with a fine toothed comb and your speedometer may be checked forensically for 'witness' marks and accuracy.

(For those wondering, a witness mark is a minuscule mark left on the speedometer facia by the needle upon impact)

 

I also inderstand @MartiniB's convoluted argument that the speedo in your Superb may not already be 100% accurate. Most manufacturers err on the side of safety. My digital speedometer displays 32 mph at a true 30.0 mph and displays 30 mph at a true 28.9 mph, as checked with LADAR.

 

As mentioned by @john999boy you are most likely to come unstuck if your car is involved in an insurance claim and is examined by an insurance assessor. Their job is to assess, and where possible reduce, the financial liability of the insurer and having tyres fitted of a size not recommended by the manufacturer would be one of their easily spotted 'Get out of Jail' cards. You could negate this by informing your insurer (as you probably should, as it's effectively a modification that puts the car outside of the manufacturer's specification) but in all likelihood they aren't going to be exactly positive about it.

 

Finally there's the ambiguous question of the validity of the warranty if you've gone away from the manufacturer's tyre recommendation.

 

It's up to you if you feel the financial saving is worth the risk. I wouldn't. It's Mea Culpa if it goes wrong!

 

I do agree about those insufferable P7 'eco' tyres. After two years there's still plenty of mileage left on them but I feel enough is enough and am about to switch them to quieter and grippier Pirelli P-Zero - but they will be the recommended size :).

Edited by BillyJim
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different sizes comes already from factory, what means "approved by manufacturer"

2.0TSi	217hp	Diameter
215/55R17	26.3"	0%	****
235/45ZR18	26.3"	0%
235/40ZR19	26.4"	+0.4%
255/35ZR19	26.0"	-1.1%
245/35ZR20	26.8"	+1.9%

2.0TSi	276hp 4x4
255/30ZR20	26.0"	-1.1%

1.4TSI	123hp
215/60R16	26.2"	-0.4%

1.4TSI	148hp ACT
215/50R18	26.5"	+0.8%

1.8TSI	177hp
235/50R17	26.3"	0%

2.0TDi	188hp
235/35ZR20	26.5"	+0.8%

 

can't see any crime by install some other combination which still stays with in 2%

225/55R17	26.7"	+1.5%
235/50R17	26.3"	0%
245/40R19	26.7"	+1.5%

 

and i have another reason to install bigger tires -

i have unique Superb with rattling and some why lowered(~2cm) suspension from the factory

look and compare by centers of wheels

my with DCC

20170613_Michelin_1_p50.thumb.png.3fbf4e1f5bfbf323c1d5e954cc9259f6.png

 

other with DCC

59690b1ebb0af_19_Sirius_Superb_III_on_White_rear.png.5d815d947ce3b8eea3ed1aad9788cb43.png

 

another, guessing without DCC

20170714_Superb_III.thumb.png.ac34a135c26a0b0b0248cb2a6ffa813b.png

 

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@BillyJim I understand perfectly that the speedometer is inaccurate but assuming the GPS is correct is also a fallacy.  The error may be less but there is still an error.  What @MartiniB appears to be commenting upon is that the introduced error due to changing tyre size is picked up by the speedo indicating a different speed.  Whereas the GPS should be the one to change.  

 

If you drive to a constant GPS speed then with an increased tyre diameter the speedo should read lower as the revolutions per mile are fewer.

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8 minutes ago, Bud said:

If you drive to a constant GPS speed then with an increased tyre diameter the speedo should read lower as the revolutions per mile are fewer.

that big answers to confirm what i have said :)
with 215/55r17 speedo shows +5km on 90km/h
with 225/55r17 speedo shows +2km
YES, lower value as before, but at same time that means, more accurate

 

p.s.
a policeman doesn't read your speedometer by his radar,

but measures car changes in space in fixed time period, GPS works the same way

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I didn't say more or less accurate.  A stopped clock will tell you the correct time twice per day but it is not accurate.

 

To look at it from the other side when you drive to the speedo @90 kmh:

 

If your speedo is under reading 90 kmh and you put on larger tyres then you are now further away from the truth but still showing 90 kmh, it is the GPS reported speed that has changed, increased, as you have not recalibrated the car system to reflect the larger rolling circumference.

 

If your speedo was over reading 90 kmh and you put on larger tyres then again the speedo doesn't change but the GPS reading increase would close the gap. 

 

Two different methods both have errors and cannot be directly correlated

 

GPS speed and police radar do not operate on the same principles.

 

A GPS has a range of errors and will refresh at about 1Hz.  Using the positional change between samples errors in those positions, jitter, will manifest as errors in calculated speed.  Hence for aircraft an embedded GPS/INU is used which uses the benefits of both systems, INU for short term and GPS for the medium term to ensure best possible accuracy.

 

Police radar has no idea where you are in space.  But uses doppler shift of the reflected energy and is therefore extremely accurate, the operating frequency for police radar is the GHz range so resolution is very accurate.

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I bet the OP wishes he hadn't asked...:sleepy:

 

1. GPS speed accuracy is dependent on many entities - signal strength, internal clock frequency, number of satellites detected (aka channels, where a minimum of four are needed before getting near any kind of accuracy of speed), vehicle movement (where elevation and direction changes are detrimental), and I don't doubt there are other factors in play. There will be times when it's accuracy will be +/- 0.1 mph, whilst at other times and circumstances it could be as much as +/- 10 mph.

2. Police radar? Correct, @Bud. Radar, though now mostly replaced by Ladar, uses the doppler effect giving an instant and constant reading of the speed of a target approaching (or receding from) the device until the operator releases the button and locks the reading. It in itself is actually only 100% accurate if the target is approaching head-on at 0º. If you start introducing angles the reading will be lower than true speed, reducing ever greater as the angle increases. The "sine angle" effect has never been in contention as it works in the motorist's favour. And in UK law the measuring device is only corroborating the officer's estimate!

3. @Bud, my "understanding" of MartiniB's comments is that in the real world +/- 1 mph at 70 mph isn't going to make any significant difference. I wasn't referring to the GPS guff. :wacko:

4. @MartiniB - Approved by manufacturer are those tyre sizes listed inside your fuel cap cover and as registered by the manufacturer at TüV. It's irrelevant which tyre sizes you think are acceptable. What matters is what the manufacturer, insurance and enforcement agencies of each respective country think is acceptable.

 

I herewith bow deeply and having submitted my penny's worth, whether right or wrong, I retire from this conversation.

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Looks like the consensus responses suggest there'll be minimum impact to the car's ride/handling. 

I am not too fuss regarding the speedo's accuracy, but will have to contact insurance to make sure that they are happy to cover the car with the new tyre size. 

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i have a superb 162tsi now, but with my previous car, a mk2 octavia rs - i wanted to change from the std 225/40/R18 to 225/45/R18, in an effort to get slightly chunkier tyre sidewalls.

but the tyre fitter (in my case Bob Jane Tyres) said no deal - they won't put on tyres sizes that r not approved for the car!!

 

so even if the issurance company clears it, u need to find a tyre shop that will supply and fit a size that is not approved.

 

i personally like what u r doing, and when it comes time to change my tyres, i would probably look at that as a option if it means i save hunderds of dollars :)

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1 hour ago, JR RS said:

i have a superb 162tsi now, but with my previous car, a mk2 octavia rs - i wanted to change from the std 225/40/R18 to 225/45/R18, in an effort to get slightly chunkier tyre sidewalls.

but the tyre fitter (in my case Bob Jane Tyres) said no deal - they won't put on tyres sizes that r not approved for the car!!

 

so even if the issurance company clears it, u need to find a tyre shop that will supply and fit a size that is not approved.

 

i personally like what u r doing, and when it comes time to change my tyres, i would probably look at that as a option if it means i save hunderds of dollars :)

 

Good point, I make sure I'll ask the tyre fitter too.

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  • 4 months later...

Just giving some updates on an old thread.

Finally decided to take the plunge and change the tyres since there was a Black Friday tyre sales a few days ago. Ended up getting the Michelin Pilot Sport 4S (apparently the Super Sport has been discontinued and this is the replacement). 

For less hassle with insurance / road regulation etc, I decided to stick with the 235/40/R19. Will be getting it fitted soon, will report back when I have a good drive of the new tyres.

The Pirelli P7 actually had hardly any wear on them even after 13,000km or so. Bit of a waste to throw them out imo, might just keep it in the shed for future usage. 

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I must admit this numpty thought that the OP was only increasing the width of his tyre, not the diameter or circumference :blush: but no doubt I missed something.

Edited by Danny 57
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47 minutes ago, Danny 57 said:

I must admit this numpty thought that the OP was only increasing the width of his tyre, not the diameter or circumference :blush: but no doubt I missed something.

 

Radial ply tyre sizes are shown as aaa/bb/Rcc dd e, where...

aaa is the tyre width in mm.

bb is the height of the tyre wall expressed as a percentage of the tyre width aaa.

R signifies radial ply construction.

cc is inner rim diameter in inches.

dd is an indexed load rating,

e is an indexed speed rating.

 

So, as you increase tyre width you increase the size of the sidewall, which in turn increases diameter and circumference...

A 235/40/R19 tyre has a sidewall 40% of the 235 mm tyre's width, which is 94 mm. Therefore diameter (19" + sidewall) is 671 mm and circumference is 2107 mm.

A 245/40/R19 tyre has a sidewall 40% of the 245 mm tyre's width, which is 98 mm. Therefore diameter (19" + sidewall) is 681 mm and circumference is 2132 mm.

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People always get quite het up about this subject but invariably neglect to realise that when comparing 'one size up/down', there are many factors that mean arguing the toss over a few mm of rolling radius is literally pointless.

 

You think the '235' & '40' on the tyre are precise to six decimal places? Of course they're not, it could in reality be 232.8 mm or 236.4 mm, 39.1% or 40.9%, and that's before you get into the inevitable difference between brands/models/speed ratings/construction/run-flats/summer/winter etc., or indeed manufacturing variability between tyres of the exact same type. Makes definitive assessment of nominal diameters between similarly sized tyres a bit pointless.

 

Inflation pressure and load will affect the effective size of the wheel/tyre combination and could easily offset or amplify the ostensible difference between adjacent sizes. Don't forget that given identical load the deflection on a wider tyre will generally be slightly less (the contact patch will stay roughly the same area).

 

And what of the difference between new and worn tyres? That's up to 6mm or so of rolling radius right there. Cars handle this kind of variation (and we don't re-calibrate our speedos as tyres wear...) and we all seem to get along just fine.

 

Of course, going more than a simple size up/down can require a bit more care, and as the OP has wisely observed, there may be regulatory considerations, but in general there isn't a lot of point getting worked up about something that results in an nominal RR within 2% of an OEM fitment to the model.

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Here's my initial impression of the Michelin Pilot Sport 4S tyres.

The tyres are much more slim looking, there's less lip to guard the rims. The overall "velvet wall" look is quite nice as it accentuate the large rim a lot more than the Pirelli P7. Although, more care must be taken now as gutter rash can occur quite easily due to the lack of lip. Surprisingly, out of the 4 Michelin PS4S tyres I got, 2 were made in USA and 2 were made in France, not sure if there'll be any quality difference. 

The steering is a bit lighter and feel more responsive. The car feel a bit more nimble due to the more direct feel of steering. However, you do feel more of the road surface flaws via the steering wheel.

The ride is also a little bit harder than the Pirelli and this translate to slightly more vibration and hardness in the cabin than previously. I guess this is due to the tyres being more "sport" orientated rather than "comfort". 

The braking of the new tyres are incredible, it is MASSIVE difference. it feels like you've upgraded to Brembo brakes, it bites hard. This is crucial for me as I do a fair bit of highway driving daily and good braking can be the difference between having an accident or avoiding one.

The PS4S is not the quietest of tyres, it is quieter than the Pirelli at high speed. The Pirelli seems to have a "droning" effect at high speed that the PS4S does not have. The noise is similar if you are driving around town. 

I have not managed to test the grip as I want to let the tyres settle down a bit more before going nut on it. 

Havent had any opportunities to test in soaking wet condition yet as it is quite dry here at the moment.

So far if you're looking for more "sporty" feel, the PS4S will not dissapoint. However, comfort is not really the tyre's forte, and sadly it does not solve the road noise issue. (I guess that is actually is due to the Superb being less noise insulated than the more expensive counterparts).

 

Will report more findings with more usage.

 

 

Edited by cyberkank
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  • 5 weeks later...

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