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Thread: Tyre sizes and the comparisons

  
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    Tyre sizes and the comparisons

    Just getting myself another set of 18" wheels to replace my current Mille Miglias. Now at the moment I'm back on my old 15" alloys which are wearing 195/60x15 Continentals.
    After searching for a cheap source for some 215/35VR18 Pirelli Pzero Nero tyres I saw some Federal 215/40VR18 SS595 tyres which look to be not only cheap but have a fairly good write up, great grip etc but no mention of performance in the wet. As tyres are very important to me I haven't decided to purchase them yet, but at £146 delivered for 2 I've got to say I'm tempted. Anyway the tyre size didn't seem quite right so I decided to use my trusty tyre size calculator website and see what the result of fitting 215/40VR18 tyres would be on the old speedo.

    Here's where it gets interesting, I input my existing tyre size, 195/60x15 radius is 307.5mm, radius of 215/40x18 is 314.6 giving a 2.309% difference and resulting in the speedo reading 71.62MPH at 70MPH. Ok I thought I can live with that. No figures were given for my existing tyres however 235/35x18 are radius 310.85 with a 70.76mph and 245/35x18 are 314.35 and 71.56mph.

    If these figures are correct why do we stick with 215/35x18 tyres and not try other sections etc or am I missing something fundemental.

    I can source cheaper tyres in some of the other sizes, what I'm asking is should I stick with what known to work or experiment a bit.
    52 Astra G 1.8, 18" Mille Miglia alloys, 325mm Brembo 4 pot conversion, Coupe leather interior, Raid strut brace, Whiteline rear ARB, Irmscher bits (skirts, Carbon steering wheel, Bonnet extension, M3 mirrors, boot & roof spoiler), Reiger Rear Diffuser & Front Bumper, Koni Adj. dampers, 35/30 Vogtland springs, Duplex quad exit ASCAR stainless exhaust.

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    Daft as it sounds (and I don't know how accurate this is) but...

    For every inch increase in wheel diameter increase the width by 10mm and decrease the aspect ratio by 10.

    So

    195/60 15
    205/50 16
    215/40 17
    225/30 18 (although I doubt you could get these!)

    'should' all be (roughly) the same rolling radius.

    I was told this ages ago by a mechanic friend but to be honest, I'm not so sure so I'll let you make your own minds up!

    Oh, and under standard arches, you'll probably not get a bigger tyre than 215/35 18 anyway - even the 215/40 18's may rub a bit
    2006 Astra VXR on 19's in Arden Blue
    ExhaustsUK Catback, Eibach Sportlines, DS Ultima, Roose hose kit, Forge recirculating dump valve, Full self modded SMD conversion

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    Quote Originally Posted by Gnasher

    195/60 15
    205/50 16
    215/40 17
    225/30 18 (although I doubt you could get these!)
    Not always true. My mk3 Golf GTI had 205/50/15's on it and my 888 has 235/40/17's on it. My mates Leon Cupra has 225/45/17's on it.
    Fastest, Most Powerful & Only Mag Featured Astra In The AOC Cumbrian Region

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    Ok anyone know the answer to this question.

    I currently use 215/35VR18 tyres.

    However they are a bitch to find sometimes and the cheapest quote I've had for Pirelli Pzero Nero tyres in that profile is £125 fitted. Which TBH isn't bad.

    However I've seen some 225/35VR18 Federal SS595 for £170 delivered a pair and they ain't got a bad write up in reviews. What's the difference in the tyre size in mm and is it just the tyre width that would alter here, my current tyres are on 7J rims but the new ones will go on 7.5J rims so maybe a wider tyre would be better to avoid the stretched look.

    I can also get 215/40VR18 Federal SS595 for £146 delivered a pair. Same width I'm thinking but higher profile which if I'm honest I'd would like as the 35 profile is very thin and I'm certain it contributed to my last rims being flat spotted as the tyre gives little or no protection against potholes etc. How much higher would these be than my present tyres as I'm fairly certain they would be ok in my arches as I'm only dropped -35mm and don't intend any further drop at any time.

    Any help guys?
    52 Astra G 1.8, 18" Mille Miglia alloys, 325mm Brembo 4 pot conversion, Coupe leather interior, Raid strut brace, Whiteline rear ARB, Irmscher bits (skirts, Carbon steering wheel, Bonnet extension, M3 mirrors, boot & roof spoiler), Reiger Rear Diffuser & Front Bumper, Koni Adj. dampers, 35/30 Vogtland springs, Duplex quad exit ASCAR stainless exhaust.

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    Scott - Which bit isn't always true?

    I was just using those sizes to support a theory on tyre sizes.

    You can't pick 3 random cars and compare the tyre sizes and assume the rolling radii are all identical?

    Don't get what you are saying - I'm not implying that these are the only sizes available in the world (ever) - Its just an example!
    2006 Astra VXR on 19's in Arden Blue
    ExhaustsUK Catback, Eibach Sportlines, DS Ultima, Roose hose kit, Forge recirculating dump valve, Full self modded SMD conversion

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    Ok found another site which says my existing 195/60x15 tyres are 615mm. Inputting a 215/35x18 tyre says that is 608mm with a -1.14% difference.
    A 215/40x18 is 629mm with a 2.28% difference but this was interesting, a 225/35x18 is exactly the same as my existing 195/60x15 at 615mm with a 0% difference.

    If that's the case why do we all choose the 215/35 section tyre? is it the rim section that dictates it? would a 225 fit better or worse on a 7.5J rim.

    All these questions and no answers
    52 Astra G 1.8, 18" Mille Miglia alloys, 325mm Brembo 4 pot conversion, Coupe leather interior, Raid strut brace, Whiteline rear ARB, Irmscher bits (skirts, Carbon steering wheel, Bonnet extension, M3 mirrors, boot & roof spoiler), Reiger Rear Diffuser & Front Bumper, Koni Adj. dampers, 35/30 Vogtland springs, Duplex quad exit ASCAR stainless exhaust.

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    Quote Originally Posted by Gnasher
    Scott - Which bit isn't always true?

    I was just using those sizes to support a theory on tyre sizes.

    You can't pick 3 random cars and compare the tyre sizes and assume the rolling radii are all identical?

    Don't get what you are saying - I'm not implying that these are the only sizes available in the world (ever) - Its just an example!
    Sorry I was posting in a rush yesterday and didnt write it correctly and now I've forgotten what I was meaning (old age) ... just disregard me lol.
    Fastest, Most Powerful & Only Mag Featured Astra In The AOC Cumbrian Region

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    these tyre profile things confuse me too...whats 215 and 225...?? i thought the profile 30, 35 etc is a ratio for the distance from the edge of the tyre to the rim at any given point...


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