Sorry if this thread has been covered before but I couldn't find any information on here, 

I have bought a set of 20" X 8 ET 41 Skoda Vega alloys which I wanted to fit to my superb mk 2 FL I'm not sure if I'll get away with 235/35/20 or maybe I'll have to go with 225/35/20 if anyone has any experience I would be very grateful. Thanks

I think you'll need 235/30/20's to keep the speedometer correct. That's what I was thinking about fitting to mine.

Thanks for the reply I've been on willtheyfit.com and the Speedo reading isn't that much out and I'll actually be going slower than it shows so I'm not to fussed about that I'm more wanting to know if the 235 would foul. Thanks again for your time

41 minutes ago, Tulseface said:

Thanks for the reply I've been on willtheyfit.com and the Speedo reading isn't that much out and I'll actually be going slower than it shows so I'm not to fussed about that I'm more wanting to know if the 235 would foul. Thanks again for your time

If your aspect ratio is higher then the circumference will be also which means your wheels will rotate slower (for a given road speed) indicating a lower speed on the speedo.

I think I got the logic of that correct.

As the speedo will be reading low (they always do) you may actually correct it but you'll need to be careful as you'll have no margin for error on those speed cameras.

 

Good luck with your fillings!

This was what it said on willtheyfit.com

1 hour ago, Tulseface said:

This was what it said on willtheyfit.com

So the speed reading is low, so for a given reading you will be going faster than before?  But 1.4 MPH at 30 and 2.8 at 60 will probably mean your speedo is closer yo correct. 

31 minutes ago, Gdcobra said:

So the speed reading is low, so for a given reading you will be going faster than before?  But 1.4 MPH at 30 and 2.8 at 60 will probably mean your speedo is closer yo correct. 

I read it as the Speedo is showing 30mph but the car is actually doing 28.59mph which means it's going slower right ?

It is saying that when you are doing an actual 30 mph, your speedo will only displaying 28.59 mph with the new tyres. That is why the chart says the new tyres will result in a -4.69% lower speedo reading.

 

That's why @Gdcobra was warning you about the speed cameras. You'll think you are safe doing 30 mph on your speedo, but you will be over that.

Yeah I've just been looking into it a bit more my bad thanks for your time everyone

1 hour ago, TheRobinK said:

It is saying that when you are doing an actual 30 mph, your speedo will only displaying 28.59 mph with the new tyres. That is why the chart says the new tyres will result in a -4.69% lower speedo reading.

 

That's why @Gdcobra was warning you about the speed cameras. You'll think you are safe doing 30 mph on your speedo, but you will be over that.

Actually that’s not quite how I read it, 

To me it is saying “When you are moving at a speed that the standard tyres are indicating as 30, with the larger tyres the indication will be 28 (in round figures).

this is not actually as bad as it seems as when you have an indicated speed of 30 on the standard tyres your true speed will probably only be 28 or so.

VAG seem to use a speedometer offset of 5 % i.e. the speedometer shows a reading that is 5 % higher than your actual speed. So at an actual speed of 30 mph, the speedometer will show 31.5 mph (30 x 1.05). This offset is not applied to the odometer however, so that will read true if the tyre size is correct.

 

The OP's example now applies an additional offset of -4.69 % to the 31.5 mph which results in a readout of  30.02 mph (31.5 x 0.9531) at an actual 30 mph. BUT, now the odometer is under-reading by 4.69 % so after an actual 10000 miles, the odometer will have clocked up 9531 miles using this tyre.

 

There's a complicating factor that willtheyfit don't account for either in that the rolling radius of the tyre as it sits under the car is smaller than its inflated radius when unloaded. This page has a tyre size calculator that accounts for this: https://www.carbibles.com/calculators/

 

The instrument cluster computes the speed by counting the number of wheel revolutions per minute and multiplying that by the circumference to get the speed (there's a calibration programmed into the instrument cluster for this). This is why a change in the tyre circumference will cause calibration issues: the actual circumference is different to the built-in calibration. On older models, it used to be possible to fine-tune the calibration in the cluster but that's not possible on the Superb 2. There are two built-in calibrations, one for the standard sizes, and a second that from what I've been able to tell matches to 235/45/R18: this is a coding option in the instrument cluster that can be changed using VCDS. If you're struggling with tyre profile issues on the new wheels, this might be an option.

 

Using the CarBibles calculator to compare 235/45/R18 to 235/35/R20 gives an error of -0.57 % which is close enough for the real world.

 

(I've written in a bit more detail about some of this in the thread below:

 

 

You must have got immaculate roads with no bumps or potholes in the Northwest.

Is this going to be a show car? How much rubber is there before you are going on the rim?

No show car it's my daily driver I just think with the size of the car it's a disgrace it being on 17" wheels what were Skoda thinking I mean look at the Audi A7 with 21" wheels on it that's more in proportion I'm going to love these 20" wheels but I just need to get things right

The stock 18s with 225/40 are firm to say the least - I suspect those 20s are going to be verging on uncomfortable on anything less than perfect surfaces.

. 

Edited 12 May, 20197 yr by jafo

On ‎09‎/‎05‎/‎2019 at 12:02, chimaera said:

VAG seem to use a speedometer offset of 5 % i.e. the speedometer shows a reading that is 5 % higher than your actual speed. So at an actual speed of 30 mph, the speedometer will show 31.5 mph (30 x 1.05). This offset is not applied to the odometer however, so that will read true if the tyre size is correct.

 

The OP's example now applies an additional offset of -4.69 % to the 31.5 mph which results in a readout of  30.02 mph (31.5 x 0.9531) at an actual 30 mph. BUT, now the odometer is under-reading by 4.69 % so after an actual 10000 miles, the odometer will have clocked up 9531 miles using this tyre.

 

There's a complicating factor that willtheyfit don't account for either in that the rolling radius of the tyre as it sits under the car is smaller than its inflated radius when unloaded. This page has a tyre size calculator that accounts for this: https://www.carbibles.com/calculators/

 

The instrument cluster computes the speed by counting the number of wheel revolutions per minute and multiplying that by the circumference to get the speed (there's a calibration programmed into the instrument cluster for this). This is why a change in the tyre circumference will cause calibration issues: the actual circumference is different to the built-in calibration. On older models, it used to be possible to fine-tune the calibration in the cluster but that's not possible on the Superb 2. There are two built-in calibrations, one for the standard sizes, and a second that from what I've been able to tell matches to 235/45/R18: this is a coding option in the instrument cluster that can be changed using VCDS. If you're struggling with tyre profile issues on the new wheels, this might be an option.

 

Using the CarBibles calculator to compare 235/45/R18 to 235/35/R20 gives an error of -0.57 % which is close enough for the real world.

 

(I've written in a bit more detail about some of this in the thread below:

 

 

What a relief to find out that my indicated digital speedo when showing 177mph was actually only doing 168mph plus the 5% correction taken into account.

12 hours ago, chimaera said:

The stock 18s with 225/40 are firm to say the least - I suspect those 20s are going to be verging on uncomfortable on anything less than perfect surfaces.

It's all about the style.

11 minutes ago, shyVRS245 said:

It's all about the style.

Absolutely right 👍

Do you have any pictures of the wheels as I'd be intrested to see them. I tried 20's on mine with no tyres and I think the max size you could go would be 235/30/20. Mine is a pre FL but I don't think that would make any difference?

Edited 12 May, 20197 yr by Big Lee

32 minutes ago, Big Lee said:

Do you have any pictures of the wheels as I'd be intrested to see them. I tried 20's qon mine with no tyres and I think the max size you could go would be 235/30/20. Mine is a pre FL but I don't think that would make any difference?

They are these wheels I have 19" on it at the moment and there's still quite a bit of space as you can see

I'm guessing you are on standard ride height? I'm on Bilstein coilovers and dropped by about 50mm. I forgot and that's why i said the tyre size I did 🤦‍♂️

1 minute ago, Big Lee said:

I'm guessing you are on standard ride height? I'm on Bilstein coilovers and dropped by about 50mm. I forgot and that's why i said the tyre size I did 🤦‍♂️

I am at the moment but I have got 30mm ebach springs to go on depending on what tyres I can get away with

22 hours ago, Tulseface said:

No show car it's my daily driver I just think with the size of the car it's a disgrace it being on 17" wheels what were Skoda thinking I mean look at the Audi A7 with 21" wheels on it that's more in proportion I'm going to love these 20" wheels but I just need to get things right

Easy now, 16's here - they have never looked out of place to me and a set of 4 crossclimates are £300 fitted. Better for your suspension, better for your bearings and a better ride. 

9 minutes ago, Shaunieboy said:

Easy now, 16's here - they have never looked out of place to me and a set of 4 crossclimates are £300 fitted. Better for your suspension, better for your bearings and a better ride. 

You forgot better for your backside also.

11 hours ago, shyVRS245 said:

You forgot better for your backside also.

A bonus for sure