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#232305 - 18/08/10 07:10 PM Re: Tyre size [Re: H47PER]
ROB IS THE STIG Offline
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Registered: 21/05/06
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Loc: Kent
Sat nav's are GPS so will be accurate surely
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#232306 - 18/08/10 07:14 PM Re: Tyre size [Re: ROB IS THE STIG]
Adam08 Offline
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Registered: 15/10/08
Posts: 15118
Loc: Harpenden, Hertfordshire
GPS is correct, speedo is not
The speedo is allowed to be up to a certain % out
Also the tyre wear changes the speedo reading
If you wheelspin, the speedo jumps up, GPS doesn't, although this isn't really relevant
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#232307 - 18/08/10 07:15 PM Re: Tyre size [Re: H47PER]
justin Offline


Registered: 30/03/10
Posts: 128
Loc: Monyash, Nr Bakewelll
Never trusted The speedo just because there is alot of tyre when new as to worn. I also belive speedo's show you going faster in general than you are actually going just to keep you legal. But i have had Three sat navs, and on my phone and they always show the same speed., 2 or 3 mph slower than speedo

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#232308 - 18/08/10 07:17 PM Re: Tyre size [Re: Adam08]
Duncan_P Offline
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Registered: 09/11/09
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I'm sure I read somewhere that car companies always put their speedo's a few mph over what speed you're actually doing. No idea why...
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#232309 - 18/08/10 07:18 PM Re: Tyre size [Re: Duncan_P]
justin Offline


Registered: 30/03/10
Posts: 128
Loc: Monyash, Nr Bakewelll
Quote:

I'm sure I read somewhere that car companies always put their speedo's a few mph over what speed you're actually doing. No idea why...

to keep you legal

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#232310 - 18/08/10 07:24 PM Re: Tyre size [Re: H47PER]
Tez Offline


Registered: 03/01/09
Posts: 2509
Loc: Co, DURHAM
Quote:

if you have 17 inch rims, they wont fit on

lol i meant get 16" wheels and that profile tyre.
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#232311 - 18/08/10 07:49 PM Re: Tyre size [Re: Tez]
justin Offline


Registered: 30/03/10
Posts: 128
Loc: Monyash, Nr Bakewelll
Ok, so i get the impression it makes no difference to the car other than the speedo. I also get the impression that not many have gone down this route. Are there any pics out there with different profile's other than 40s on 17inch alloys. Just trying to figure out if it would make an improvment to the look

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#232312 - 18/08/10 07:50 PM Re: Tyre size [Re: justin]
Micky Offline
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Registered: 01/03/05
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It will also affect the mileage of the car as that's tied into the speedo
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#232313 - 18/08/10 07:58 PM Re: Tyre size [Re: Micky]
Dan_The_Man Offline
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Registered: 20/10/08
Posts: 7389
Loc: Eaton Bray, Dunstable
it would also make the ride less harsh im sure!
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#232314 - 19/08/10 11:20 PM Re: Tyre size [Re: Duncan_P]
ChrisCumbria Offline

Driving as Daily

Registered: 26/06/05
Posts: 23756
Loc: Whitehaven, Cumbria
Quote:

I'm sure I read somewhere that car companies always put their speedo's a few mph over what speed you're actually doing. No idea why...



In simple terms, to make sure that the manufacturer stays within the law. Here's why:

The law allows speedometers to over-read by up to 10% but not to under-read at all. The problem therefore if they set them up calibrated to read the correct speed, is that - assuming the conditions on the production line are the same throughout any batch of speedos being made - statistically 50% of them would under-read by up to whatever the manufacturers tolerance is. Since legally they can't under-read at all this is obviously no good!

Therefore to get round this then effectively they are deliberately wrongly calibrated by shifting the scale on the dials upwards. What this means in practice is that even the 50% that are "under" reading are still reading higher than the true speed, and are therefore complying with the law.

At this point I could start getting really geeky by talking about confidence intervals, whether the range of uncertainty in the speedometers has a normal distribution etc etc, but I think I'll leave that for another time!
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#232315 - 05/09/10 08:03 PM Re: Tyre size [Re: justin]
SnowmanST Offline


Registered: 03/09/10
Posts: 10
So the only difference would be the speedo ??? what about 215/45/17 ???

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#232316 - 05/09/10 08:06 PM Re: Tyre size [Re: SnowmanST]
justin Offline


Registered: 30/03/10
Posts: 128
Loc: Monyash, Nr Bakewelll
205 is tyre size from front to back 40 is from tread to alloy rim and 17 is the diameter of the alloy itsself

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#232317 - 05/09/10 08:08 PM Re: Tyre size [Re: justin]
justin Offline


Registered: 30/03/10
Posts: 128
Loc: Monyash, Nr Bakewelll
its tread of tyre to the rim or the wall of the tyre that needs changing, ie 40 to 45 or more

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#232318 - 07/09/10 04:03 PM Re: Tyre size [Re: justin]
SnowmanST Offline


Registered: 03/09/10
Posts: 10
So Did you fit the 205/45/17 then?

Cheers

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#232319 - 07/09/10 07:13 PM Re: Tyre size [Re: SnowmanST]
justin Offline


Registered: 30/03/10
Posts: 128
Loc: Monyash, Nr Bakewelll
Not yet, Waiting until winter when fresh tyres will give me a bit of grip in the snow

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