Low profile to high profile tyre, 20" to 18"

[WakeFlakes]

Hi Guys,

So I have recently got an FK8 Type R, I'm paranoid about curbing the wheels due to the low profile tyres. I have heard these wheels can be a ballache to get refurbed due to the red piping. So I am thinking about downsizing the wheels from a 245/30R20 to a 245/40/R18 or a 235/45/R18. Would this result in higher profile tyre to help avoid kerbing the wheel? Or would an even larger sidewall be required?

I don't think there is a high profile tyre that I could fit on the OEM wheel? I would potentially be looking at replacing the tyres anyway, as they are cheapo no name brand that seem to produce a lot of road noise.

Many thanks in advance for your advice.

[Scrump]

If you fit a higher profile tyre on the oem wheel (I presume you mean the 20 inch) then you will be raising the gearing and incur a speedo error.

For checking what tyre sizes on smaller wheels will give the same overall diameter then I use the ‘tire size’ app. Put in original and new tyre and wheel size and it gives you all the required info.
If you go to 18 inch wheels the. Check they will fit over your brakes.

Are any alternative wheel/tyre sizes listed in the handbook or on the tyre pressure sticker?

Edit: ran your tyres sizes through the app. 245/30R20 to a 245/40/R18 would give identical circumference, 235/45/R18 would give a 2% difference in circumference so probably also okay.

Edited by Scrump on Thursday 28th April 09:30

[snotrag]

https://www.willtheyfit.com/

[DodgyGeezer]

try this:

https://tiresize.com/calculator/


type in your current size and then what you're thinking of.....

[WakeFlakes]

Thanks guys, I have checked on the calcs and I know the tyres will fit. However I was more asking in the bigger side wall would protect the wheels enough or if they would still be too low profile.

[Scrump]

To some extent that depends on the wheel width versus tyre width, not just on sidewall height.

[trevalvole]

WakeFlakes said:

Thanks guys, I have checked on the calcs and I know the tyres will fit. However I was more asking in the bigger side wall would protect the wheels enough or if they would still be too low profile.

Going from 245/30R20 to 245/40R18 gains you about an inch in tyre sidewall height. You can also get tyres with additional rubber on the sidewall to help protect the wheel which is known as rim protection.

[Pica-Pica]

To begin with. Rolling circumference is
245/30 R20 gives you 2001mm
245/40 R18 gives you 1992mm
235/45 R gives you 2040mm.
With the 2040mm you may find that will cause the engine to hunt between gears on a slight incline. That is very annoying (I am considering an automatic with 8 or 9 gears).
For normally laden sidewall heights (that is the ground side of the tyre) you will have
64mm, 88mm, and 96mm respectively. You really need 110+mm to avoid kerbing and rim protectors only help a little (I know, by experience).
What may help is to go to the narrowest wheel rim width that will take your chosen tyre. That will push the sidewall further away from the rim edge.
The reduction in rim diameter will certainly give a better ride comfort. It will help to some extent with the severity of any kerbing, but will not eliminate the possibility completely, and rim protectors (on the tyre - NOT a separate rim attachment) may reduce the severity, but not eliminate the possibility.