3 hours ago, UndertheRadar said:

I have 19” summers and 18” winters. The difference in ride quality is just about noticeable.

 

What are the official tyre pressures with the 235/40R19 and 235/45R18 tyre sizes, and are you using them?

 

You might find the official tyre pressures on a label inside the fuel filler flap.

 

Standard load index (ie. not XL)

215/60R16 95

215/55R17 94

235/45R18 94

235/40R19 92

245/45R18 96

 

Notice that the standard load index on 245/45R18 is 4 higher than 235/40R19. This means that you can use about 4psi lower tyre pressures on the 245/45R18 size compared to 235/40R19.

 

Running underinflated 19" tyres in a desperate attempt to match the comfort of slightly over-sized 18" tyres is not the answer, as the rims could be far more easily damaged by potholes, and under-inflation has an adverse effect on handling. Plus it's illegal to run under-inflated tyres and dangerous. Severely under-inflated tyres can lead to a blow-out due to overheating at high speed.

 

Edited 14 January, 20224 yr by Carlston

4 hours ago, Carlston said:

 

What are the official tyre pressures with the 235/40R19 and 235/45R18 tyre sizes, and are you using them?

 

You might find the official tyre pressures on a label inside the fuel filler flap.

 

Standard load index (ie. not XL)

215/60R16 95

215/55R17 94

235/45R18 94

235/40R19 92

245/45R18 96

 

Notice that the standard load index on 245/45R18 is 4 higher than 235/40R19. This means that you can use about 4psi lower tyre pressures on the 245/45R18 size compared to 235/40R19.

 

Running underinflated 19" tyres in a desperate attempt to match the comfort of slightly over-sized 18" tyres is not the answer, as the rims could be far more easily damaged by potholes, and under-inflation has an adverse effect on handling. Plus it's illegal to run under-inflated tyres and dangerous. Severely under-inflated tyres can lead to a blow-out due to overheating at high speed.

 

yes under inflating a tyre might help with comfort as sidewall will flex more, but that comes at a cost,  ie increased mpg and wear on the outside & inside edges of the tyre

imo better to go 17" with 55 series tyres set at standard pressures

10 hours ago, Carlston said:

 

What are the official tyre pressures with the 235/40R19 and 235/45R18 tyre sizes, and are you using them?

 

You might find the official tyre pressures on a label inside the fuel filler flap.

 

Standard load index (ie. not XL)

215/60R16 95

215/55R17 94

235/45R18 94

235/40R19 92

245/45R18 96

 

That load index for the 19" is incorrect - the minimum required load index is 96, as indicated on the fuel flap....

 

 

19 hours ago, JR RS said:

 

That load index for the 19" is incorrect - the minimum required load index is 96, as indicated on the fuel flap....

 

 

 

It's not incorrect.

 

I gave the standard load index, not the XL (eXtra Load) index.

 

I used it to compare the comfort levels and assess the tyre pressures between different tyre sizes.

 

You can't compare comfort levels and tyre pressures from one size to another, if you use the standard load index on one tyre size and the XL index on the other tyre size.

 

Going by the tyre label inside the fuel filler flap, the Superb MK3 fitted with slightly oversize 245/45R18 tyres would need 32psi (2.2Bar) front and 32psi (2.2Bar) rear with a lightly loaded car, and 35psi (2.4Bar) front and 39psi (2.7Bar) rear with a heavily loaded car. These figures are obtained by comparing the recommended tyre pressures and standard load indexes (not XL) of the 235/40R19 tyre size and the slightly oversize 245/45R18 tyre size.

 

The 235/40R19 tyre size can support 630kg when inflated to 3Bar, and the slightly oversize 245/45R18 tyre size can support 710kg when inflated to 3Bar. Bear in mind, that the weight a tyre can support is proportional to the tyre pressure. For example, halve the tyre pressure and the weight it can support is halved...and vice-versa...double the tyre pressure and the weight it can support is doubled. You can't inflate a tyre to more than it's recommended tyre pressure, and you can't support more weight than recommended by the tyre manufacturer.  

 

Edited 15 January, 20224 yr by Carlston

I disagree. 

It's simple - the fuel card states the minimum load index required, for each of the approved tyre sizes.

 

It's not about, nor states, the tyre being standard or XL.  That's the owners choice.

 

If u have 235/40/19 tyres, the minimum load rating required is 96.

Simple.

 

I can b stupid n get a dirt cheap 235/40/19 tyre with a load index rating of 92, which fails to meet the minimum requirements, hence a defect, and making insurance void.

 

Or I can buy same sized tyre, with the correct load rating of 96, for double the price (almost).  

 

 

I also don't understand why u keep referring to the Superb being fitted with the 245/45/18 tyres - it's not. Standard is 235/45/18, as shown in the fuel card too.

Yes u can fit 245/45/18s, but it's not the standard, nor common.

 

I've increased the tyre profile on mine by fitting 245/40/19 (96 XL) instead of the standard 235/40/19.

Edited 15 January, 20224 yr by JR RS

@JR RS

btw. 245/45/18 and 245/40/19 are identical in Width and Diameter

me too installed 225/55r17 instead 215/55r17

to add 10mm in Width, 5mm in Height and 1.5% in Diameter

 

https://tiresize.com/comparison/

 

18 hours ago, JR RS said:

I disagree. 

It's simple - the fuel card states the minimum load index required, for each of the approved tyre sizes.

 

It's not about, nor states, the tyre being standard or XL.  That's the owners choice.

 

If u have 235/40/19 tyres, the minimum load rating required is 96.

Simple.

 

I can b stupid n get a dirt cheap 235/40/19 tyre with a load index rating of 92, which fails to meet the minimum requirements, hence a defect, and making insurance void.

 

Or I can buy same sized tyre, with the correct load rating of 96, for double the price (almost).  

 

 

I also don't understand why u keep referring to the Superb being fitted with the 245/45/18 tyres - it's not. Standard is 235/45/18, as shown in the fuel card too.

Yes u can fit 245/45/18s, but it's not the standard, nor common.

 

I've increased the tyre profile on mine by fitting 245/40/19 (96 XL) instead of the standard 235/40/19.

 

There's many inaccuracies in your post.

 

I usually refer to the 245/45R18 as slightly oversize, never as a standard fitment because its not.

 

I haven't suggested anyone fit standard load index 235/40R19 tyres.

 

I have used the standard load index in all tyre sizes to assess comfort levels and tyre pressures, as this creates a level playing field for the comparisons.

 

Comparing a 245/45R18 96 SL (Standard Load) index tyre with a 235/40R19 96 XL (eXtra Load) index tyre would be meaningless, because the standard load index tyre gets its load index from 44psi (3Bar), whereas the XL index tyre gets its load index from 51psi (3.5Bar).

 

235/40R19 with a standard load index aren't necessarily cheap. For example, by far the cheapest tyre in that size currently listed on mytyres is an XL tyre. However, I've never encouraged the use of cheap, poor performing tyres as it's a very bad idea. In fact, I'm a big fan of top performing all-season tyres with the 3PMSF symbol that work well in all conditions.

 

From a point of safety, a 235/40R19 standard load index tyre can support the same weight as an XL tyre in the same size when they are inflated to the same pressure up to 44psi (3Bar), eg. both tyres can support the same 630kg weight when inflated to 44psi (3Bar). The XL index tyre can only support more weight than the SL index tyre when inflated above 44psi (3Bar).

 

Edited 16 January, 20224 yr by Carlston

13 hours ago, MartiniB said:

@JR RS

btw. 245/45/18 and 245/40/19 are identical in Width and Diameter

me too installed 225/55r17 instead 215/55r17

to add 10mm in Width, 5mm in Height and 1.5% in Diameter

 

https://tiresize.com/comparison/

 

 

indeed - they r both pretty much identical, with the 18" being 10 mm taller (sidewall) 

 

by me switching from 235/40/19 to 245/40/19 - i gained 5mm in sidewall height, resulting in better ride comfort

the speedometer reading is also closer to the true GPS speed now.