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Old 02-04-2009   #1
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Tyre size

Ive changed my tyre size down to a 205/40/17 on my Tjet, i was just wondering if any of you clever people can point out if this is likely to have any negative effect on my car etc apart from knocking the speedo out by another couple of mph as i have my dad moaning at me about changing from a 205/45. Any replies are greatly appreciated.
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Old 02-04-2009   #2
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Re: Tyre size

Quote Originally Posted by punto_matt View Post
Ive changed my tyre size down to a 205/40/17 on my Tjet ...
Hi Matt - not done the full number crunching, but on the face of it it's quite a step down. But another way of looking at it is - it's not a big step change in comparison to someone running on 175/65R15 (the smallest radius `book' tyre) ... when both full tread-depth new of course. I'll look at numbers later for you. What's dad's concern?
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Old 02-04-2009   #3
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Re: Tyre size

He thinks that it will invalidate my insurance as it is not an approved tyre size from fiat. He also says it will mess up my traction control but this is mainly because in my stepmums X5 they are having problems because a tyre is slightly defected but they cant find out why or how and sends the traction control mental.
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Old 02-04-2009   #4
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Re: Tyre size

I had 205/40/17 on the front of mine for a while. Had a set of toyos left on the 17" rims I had on the C2 so, i thought i may as well use them.

Had them on for a good few months and had no bother at all.
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Old 02-04-2009   #5
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Re: Tyre size

im running slightly larger on the front (215/45) and to be honest I was expecting ESP or ABS warnings, especially at higher speed but have had nothing in a few thousand miles like this.
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Old 02-04-2009   #6
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Re: Tyre size

Im not having a problem with them its just my dad that is moaning at me about them.
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Old 02-04-2009   #7
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Re: Tyre size

My 17"Linea alloys (supplied by Fiat) are on 215/40s. They have been fine no problems at all.
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Old 02-04-2009   #8
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Re: Tyre size

205/40 Diameter Difference: 3.34%

215/40 Diameter Difference: 2.02%


The limmit is 3%,so 205/40 are out of the limmit.
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Old 02-04-2009   #9
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Re: Tyre size

Quote Originally Posted by punto_matt View Post
Ive changed my tyre size down to a 205/40/17 on my Tjet, i was just wondering if any of you clever people can point out if this is likely to have any negative effect on my car etc apart from knocking the speedo out by another couple of mph as i have my dad moaning at me about changing from a 205/45. Any replies are greatly appreciated.
I'm not clever, but what you have done going from 45 profile to 40 is that you have increased the acceleration slightly, reduced the maximum speed a whiff & reduced your ride quality. You have also thrown your warranty out of the window.. Your dad was correct - they nearly always are. Call it experience of life....
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Old 02-04-2009   #10
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Re: Tyre size

I know they are out a bit but its not that much further out than with my 205/45s on. speed on my sat nav is only 1/2mph further out than when i had my 205/45s on.
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Old 02-04-2009   #11
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Re: Tyre size

Quote Originally Posted by SurfBored View Post
I'm not clever, but what you have done going from 45 profile to 40 is that you have increased the acceleration slightly, reduced the maximum speed a whiff & reduced your ride quality. You have also thrown your warranty out of the window.. Your dad was correct - they nearly always are. Call it experience of life....
Yeah because my warrenty is still intact with all the other stuff i have done. Ride quality is still the same as before, top speed not like im planning on hitting that.
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Old 02-04-2009   #12
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Re: Tyre size

all you can do really is ride it out and wait for something bad to happen.
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Old 02-04-2009   #13
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Re: Tyre size

http://www.tyres-online.co.uk/techinfo/upstepping.asp

going to a lower profile tyre would give you better handling and grip due to the side wall being stronger - as for the speedo reading being out further I'm not too sure as the reading would come from the ABS control unit which takes it from the hub rather than the gearbox.
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Old 02-04-2009   #14
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Re: Tyre size

The speed being out a bit more doesnt bother me too much it was just from the insurance side my dad was bugging me but ive checked with them and they just put them down as uprated tyres with no extra charge.
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Old 02-04-2009   #15
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Re: Tyre size

Quote Originally Posted by punto_matt View Post
Ive changed my tyre size down to a 205/40/17 on my Tjet ...
Inside the book:
I looked at rolling radius a couple of weeks back (currently running on different front & rear sizes) – but keeping within the tyre sizes given in the back of my handbook (603.81.048 I/2006 1st ed). I found that the 205/45R17 is around the mid-point for RR of the tyre sizes given – the other sizes were all within +/- 3mm of the 205/45R17 RR of 307mm (may vary - depends what calc. tool you use). Simply, whatever tyre size from the handbook - then in terms of RR, the variation wasn’t worth troubling about.

Outside the book:
Moving from 205/45R17 to 205/40R17 steps-down your RR by some 10mm (205 x 0.05). The 205/40R17 is (as you say) outside the book - smaller RR (298mm) than the smallest book size (304mm) of the 175/65R15. But then the difference is only around 6mm – then effectively on 205/40R17s you have the same RR as (say) running on worn 175/65R15s. But also the difference becomes greater over time as the 205/40R17s wear. Also initially (I think) your indicated speed will still over-read your actual - although the calc. suggests otherwise (by 2mph or so), I’d argue that’s not enough to take-up all the over-read tolerance. But then as treadwear progresses, you’ll be moving towards where indicated speed = actual speed. How close you’ll be to the (illegal) actual speed being greater than indicated will need a harder look at the numbers.

To argue your case with dad, I’d say that most in the FF wouldn’t give a negative wave to someone moving from (say) their current 175/65R15 to 205/40R17 on the grounds that the difference (actually 2.09%?) is close to the generally acceptable 2%. (Although I’m not clear where this 2% acceptance level comes from). On the insurance element, if you’ve advised them of `any changes’ since your last contract - then they can’t block you with any ABWAM (Ah – but wait a minute) should any `disappointing incident’ arise. The dominoe effect on traction, gearing etc. is outside my envelope of knowledge.

If you want to play with tyre calc’s – I tend to prefer …

http://www.carbibles.com/tyre_bible_pg2.html

I was out to lunch yesterday with an old-timer with a `57 Corsa 1.2 & noticed he was running on 185/70R14 tyres. If we’re all supposedly within the Gamma/SCCS platform – how is it there doesn’t seem to be a consensus over tyre sizes? What determines the tyre spec. of a model?
Thanks punto_matt thanked for this post
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