Increasing wheel size but not tyre width?

forty3

forty3

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I've unfortunately just discovered that I have a couple of buckled alloys so I'm thinking about an upgrade to 20s from my current 19" rotors.

Genuine 20" rotors seem to be 8.5j which is the same width as I currently have along with 255/35/19 tyres.

What effect would going up a wheel size but keeping the width the same have? I. E. using 255/30/20?
 
the first part of the tyre size 255 is the width across the tyre pal, the second part 35,30 is the sidewall height or thickness however you want to call it.
youve seen my post about fitting my 20's yesterday but from everything ive read ideal size for 20's is 245 35 20.
i bought mine already fitted with 255 35 20 so had no choice but to fit them but they fit fine with the et35 offset, personally though i wouldnt want a 30 profile with the state of english rd's, it may only be 5mm difference but its something to help the ride over bumps etc!
one other thing is it can affect your speedo by a few mph, as you go bigger alloy you should go lower profile tyre.
theres a website you can put the wheel and tyre size into and the original size and it will tell you the difference in mph,where it will stick out or sit in more etc too.
 
glenandem said:
the first part of the tyre size 255 is the width across the tyre pal, the second part 35,30 is the sidewall height or thickness however you want to call it.
youve seen my post about fitting my 20's yesterday but from everything ive read ideal size for 20's is 245 35 20.
i bought mine already fitted with 255 35 20 so had no choice but to fit them but they fit fine with the et35 offset, personally though i wouldnt want a 30 profile with the state of english rd's, it may only be 5mm difference but its something to help the ride over bumps etc!
one other thing is it can affect your speedo by a few mph, as you go bigger alloy you should go lower profile tyre.
theres a website you can put the wheel and tyre size into and the original size and it will tell you the difference in mph,where it will stick out or sit in more etc too.
Click to expand...
The difference between 35 profile and 30 profile isn't 5mm. It's 5% of the tyre width. So the sidewall of a 255/35 tyre is 89.25mm. The sidewall of a 255/30 tyre is 76.5mm, so the difference is 12.75mm.
 
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The 30, 35, 40 etc is the side wall height in % of width. So a 30 profile on a 245 is actually a bit less side wall than a 30 profile on a 255.

I think the rs4 with optional 20's has 30 profile tyres on in either 255 or 265
 
I understand how tyre sizing works but I was wondering what happens if I go from 19s to 20s but keep the width at 255. Will I lose grip etc?

Gazwould said:
How did the 19's get buckled ?
Click to expand...

No idea as bought the car like it. Noticed on the drive home that it wobbled a bit but put it down to the wheels needing to be balanced as the tyres were new. Took it to a tyre place and they showed me how badly out of true the wheels were by spinning it by hand on the balancing machine.

RyanJonS4 said:
The 30, 35, 40 etc is the side wall height in % of width. So a 30 profile on a 245 is actually a bit less side wall than a 30 profile on a 255.

I think the rs4 with optional 20's has 30 profile tyres on in either 255 or 265
Click to expand...

From what I've researched the b8 Rs4 has 265/30/20 tyres.
 
jdp1962 said:
The difference between 35 profile and 30 profile isn't 5mm. It's 5% of the tyre width. So the sidewall of a 255/35 tyre is 89.25mm. The sidewall of a 255/30 tyre is 76.5mm, so the difference is 12.75mm.
Click to expand...

I knew it was something like that over what I'd said lol, 12-13mm extra is a big help saving tyres and ride, I remember when I used to think 40 profiles was skinny they're just getting sillier now!!!
 
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255 width is 255 width wether it's on a 19" or a 20" makes no difference still same width so same contact

Only thing I would say I found it wasn't a great ride on 20s with 30 profile , I ran them a while ago and ended up going back to 19s after various buckle repairs and finally a cracked rim
 
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Who can tell , welcome to the world of reality where maths don't apply to rubber .
 
So a 255 is a 255 unless your being really anal about it
 
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Hahaha you've lost me now mate, ****** tyre manufacturers playing god who do they think they are :tearsofjoy:
 
Two 235 45 17's that belonged to a moderator here .

IMG_2741_zps7f5c9c25.jpg
 
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The tyre width thing is crazy isn't it.
My current 245's are 248's haha (quite close)
 
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Random and common , it affects profiles aswell.

I had 205 55 16 and 235 45 17 and the 235 45 17 was much taller than the math .
 
Didn't know there was such a variation in tyre widths!

Chop. said:
255 width is 255 width wether it's on a 19" or a 20" makes no difference still same width so same contact
Click to expand...

So a 255/35/19 has the same amount of contact with the road as a 255/30/20?
 
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below is the basic principle,not taking into consideration the differences between tyres explained above




tyresize.jpg
 
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Might also be worth looking at the tyre load rating.

I went from 18s to 20s for my old A5, but the 20s were from an A6 so the alloy width was 8.5j, rather than 9j. As a result I could only fit a 255/30/20 tyre, not a 265/30/20 tyre. I noticed that the tyre load rating was lower, and did wonder what impact that would have on insurance....

Not only that, but the overall circumference was different too, so my speedo was not as accurate. Others say that can have an affect on ABS and gearing.

In all, I would just get the right fitting alloy and tyres :p
 
Hahaha Jesus I'm just gona pick the black ones
 
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That's a lot of reading. Haha!

It says at the start that it's comparing two different width tyres but with the same overall diameter so doesn't answer my original question as I wanted to know what effect (if any) reducing the diameter of a tyre has on grip etc.
 
forty3 said:
That's a lot of reading. Haha!

It says at the start that it's comparing two different width tyres but with the same overall diameter so doesn't answer my original question as I wanted to know what effect (if any) reducing the diameter of a tyre has on grip etc.
Click to expand...
I know; I'm still looking for a link to info that's a bit more succinct.
 
Chop. said:
...Only thing I would say I found it wasn't a great ride on 20s with 30 profile , I ran them a while ago and ended up going back to 19s after various buckle repairs and finally a cracked rim
Click to expand...

Chop, you're a legend. I didn't know buckled wheels could be repaired util I read your comment.

Just got back from an alloy repair shop who have sorted out the 2 buckled alloys for just £60 and solved my vibration issue. You've saved me a fortune!
 
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forty3 said:
Chop, you're a legend. I didn't know buckled wheels could be repaired util I read your comment.

Just got back from an alloy repair shop who have sorted out the 2 buckled alloys for just £60 and solved my vibration issue. You've saved me a fortune!
Click to expand...


Haha your welcome mate glad your sorted
 
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One thing I'd like to add is I've had cracked wheels repaired before and they just kept on cracking (because they were weakened or something) so after buying tyres and tyres and getting repairs and repairs I ended up with a new set, hopefully this isn't the case but just thought I'd add my experiance, unfortunately this tends to be a bigger wheel trait.
 
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