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Thread: Oversize tyre question

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    Default Oversize tyre question

    Hello,

    My alloys are 17" J7.5 ET39
    I know that the factory tyre size is 225/45 R17 on the pre-facelift model. Is it possible to use 225/50 instead to reduce wheel arch gap without rubbing ? It is not a huge difference in size that's why I dared to take it into consideration. Is there anyone using that size and has first hand experience ? Thanks for your reply in advance.

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    Regular Member northpole's Avatar
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    225/45/17 is not the factory standard here it is size is 215/50/17 so unless the tire size has been changed in the ecu you already have a deviation that makes the car read faster than you are actually doing... so if you go to a size 50 (read higher tire wall) regardless of width you'll be having an accurate speed reading. width wil give you more grip on a dry road and more tire noise while driving... when it rains however you have more rubber which can easily cause aqua-planning (tire lifting of the road since the water lifts it of it) especially if the tires are loosing their profile..

    long story short you can go for wider tires without a problem if you take the above into account... turning up the music a tad louder will help with the tire noise...

    ps you missed to introduce yourself, do it here
    Last edited by northpole; 12th January 2021 at 12:57.

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    Quote Originally Posted by northpole View Post
    225/45/17 is not the factory standard here it is size is 215/50/17
    But the handbook suggests that 225/45/17 can be fitted instead of 215/50/17. Which a lot of us have previously done because the 215 cost is considerably more.

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    Regular Member john_k_sri's Avatar
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    Yes, I believe 225/45/17 was an option on Vectra 'C', and IIRC standard fit on Saab 93 ('Posh' Vectra) LOL.

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    I don’t know how much tyres are in Hungary but over here 225/45/17s are generally the cheapest then probably the 215/50/17s then the 225/50/17s are the most expensive (but not huge differences). Personally lowering springs would be better if you want to minimize the wheel arch gap. The tyres aren’t going to make a noticeable difference. The profile 45 or 50 is measured as a percentage of the width of the tyre. This means you have a %5 difference between the 225/45/17 and 225/50/17. Doing some rough maths and comparing the same tread depth, your car will in theory sit about 11mm higher off the ground and close the wheel arch by the same amount. Lowering springs make your car sit lower and close the gap

    As North Pole said it will slightly alter your speedometer but not much.

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    Regular Member northpole's Avatar
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    Quote Originally Posted by Fastcabs View Post
    As North Pole said it will slightly alter your speedometer but not much.
    It does make a difference as I said before i went from.the 215/50/17 to 225/45/17 and when I do 30 on the speedo it's actually doing 28.5 mph etc etc etc... so that 5% difference is more the faster you are going... (measured it with my gps)

    But if your car has the 215/50/17 set as standard than your speedo will be accurate with 225/50/17...

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    Regular Member northpole's Avatar
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    I can only imagibe that he mind endup with a plus 5% deviation meaning his speedo will be reading lower than he is actually doing.... but it can be set in the ecu... thus it reading fine... you'll need either tech2 or opcom for that

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    Quote Originally Posted by northpole View Post
    But if your car has the 215/50/17 set as standard than your speedo will be accurate with 225/50/17...
    speedo will still be out compared to the standard 215/50/17s. 50 profile means %50 of width. True profile in mm with a perfectly new tyre is %50 of 215mm and %50 of 225mm so 107.5mm and 112.5mm respectively. Overall diameter is going to be 10mm different between those tyre sizes not taking pressures etc into account.

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    Regular Member northpole's Avatar
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    Quote Originally Posted by Fastcabs View Post
    speedo will still be out compared to the standard 215/50/17s. 50 profile means %50 of width. True profile in mm with a perfectly new tyre is %50 of 215mm and %50 of 225mm so 107.5mm and 112.5mm respectively. Overall diameter is going to be 10mm different between those tyre sizes not taking pressures etc into account.
    Yeah you ate.rigjt bit ax gar ad I know you can set it it in the ecu and that should sort it out properly... I haven't done mine and my cable is gone

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    Well, I am on steel 195/60r15 cheese cutters. I had a good deal on a set of aforementioned Omega alloys with curb rash that I'm about to sand and repaint. I was informed on another forum that the factory 18" 225/45r18 wheel is even 2mm taller than the 225/50r17 I'm considering which means it should physically fit in the wheelarch without rubbing so I'm left with two options. According to the online tyre size calculator the diametre difference is 3.6% between the two. I haven't made up my mind yet but the taller tyre profile would come in handy knowing how bad our roads are. There is like £3 difference between the /45 and /50 so it doesn't really matter.

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