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      01-20-2013, 03:19 AM   #1
RobUK
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Question Speedo (over)reading with winter tyres

The Mrs was being teased by a friend yesterday for having fitted winter tyres to her 4x4, said friend claiming they're nothing but a marketing ploy. So, I was looking around the web for suitable reviews & stories to teach this person the facts and saw this interesting snippet on an Autocar review...

"There were some other qualities about the car on winter tyres that were less desirable, true. I noticed, for instance, that the speedo had become wildly ambitious; at a true 70mph it was reading almost 80mph, which meant the fuel range indicator was similarly off piste. And the carís traction control also become neurotic, killing the power at the merest whiff of throttle, even on bone dry roads. "

Reviewer reckoned that winter tyres are worth every penny, but I'd not heard about this effect on the speedo. Anyone confirm this?
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      01-20-2013, 03:24 AM   #2
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well it will depend what size wheels and tyres they fitted. eg if they are smaller the speedo will over read.

Personally I like the Auto Express tyre review comment...

At 25mph all the winter tyres took around 20 metres to stop on snow.

However, whilst the winter tyres were stopped, the control summer tyre was still doing 20mph.
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      01-20-2013, 03:46 AM   #3
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Quote:
Originally Posted by RobUK View Post
The Mrs was being teased by a friend yesterday for having fitted winter tyres to her 4x4, said friend claiming they're nothing but a marketing ploy. So, I was looking around the web for suitable reviews & stories to teach this person the facts and saw this interesting snippet on an Autocar review...

"There were some other qualities about the car on winter tyres that were less desirable, true. I noticed, for instance, that the speedo had become wildly ambitious; at a true 70mph it was reading almost 80mph, which meant the fuel range indicator was similarly off piste. And the car’s traction control also become neurotic, killing the power at the merest whiff of throttle, even on bone dry roads. "

Reviewer reckoned that winter tyres are worth every penny, but I'd not heard about this effect on the speedo. Anyone confirm this?
Yes, it's all about tyre sizing.

For example I'm reading that users are fitting winter wheels off of E9x models to the F30/31 and saying no 'fitting' issues. But the tyre profiles are different, so there will be speedo error.

Also some fit narrow tyres for the winter and forget they also need to change aspect ratio, to keep the correct tyre RC. For example a car with a 245/45 R18 summer wheel will need a 225/55 R17 if going for the narrow but smaller rim winter tyre.

A lot of X3 drivers are putting very strange size tyres on their models, because of tyre availability or cost. Not matching the RC at all.

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      01-20-2013, 03:48 AM   #4
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Originally Posted by jakeb1 View Post
However, whilst the winter tyres were stopped, the control summer tyre was still doing 20mph.
...and busy ploughing into the car in front

Regards speedo reading, I assumed that a winter tyre would have the same rolling radius as the equivalent summer tyre?

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Originally Posted by HighlandPete View Post
Yes, it's all about tyre sizing.

For example I'm reading that users are fitting winter wheels off of E9x models to the F30/31 and saying no 'fitting' issues. But the tyre profiles are different, so there will be speedo error.

Also some fit narrow tyres for the winter and forget they also need to change aspect ratio, to keep the correct tyre RC. For example a car with a 245/45 R18 summer wheel will need a 225/55 R17 if going for the narrow but smaller rim winter tyre.

A lot of X3 drivers are putting very strange size tyres on their models, because of tyre availability or cost. Not matching the RC at all.

HighlandPete
Ah yes, I can see how changing size would do it. Kind of got the impression from the Autocar review that they'd be using the correct sized tyre.
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      01-20-2013, 03:53 AM   #5
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...Regards speedo reading, I assumed that a winter tyre would have the same rolling radius as the equivalent summer tyre?
Yes it will if the same size, tolerance will be very similar.

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      01-20-2013, 04:59 AM   #6
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The E9X wheels will fit no problems as they have the same Centre Bore and PCD, lucky for me the ET was also the same.

Anyway I ended up having to buy a new set of tyres (Blizzak LM32 RFT) as the tyre on the F31 from the E91 was a touch bigger. We thought about just running the E91 winters but it would of altered the speedo reading by about -3mph and a lower load rating plus invalidated our insurance! So for the sake of a few hundred quid we just did it.

On another note most garages said they have noticed a lot of car makers doing this lately as it helps improve mpg values.
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      01-20-2013, 05:23 AM   #7
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What's the offset for F30/31 models?
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      01-20-2013, 05:36 AM   #8
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The E9X wheels will fit no problems as they have the same Centre Bore and PCD, lucky for me the ET was also the same.
Oh dear. Really? I'd read somewhere on here that the E9x wheels didn't fit at all due to the offset. Bugger, I sold mine when the E91 went
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      01-20-2013, 06:13 AM   #9
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What's the offset for F30/31 models?
Offset depends on what size alloy your attempting to fit, so for me the ET on the E91 and F31 was ET31. The only thing that has changed between the E91 & F31 is the bolt thread.

I checked by looking for the ET values on realoem site and calling my dealer for confirmation. The dealers of course will just give a load of bull***t about offset are wrong etc etc.....they want to make a sale.
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      01-20-2013, 12:40 PM   #10
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Is there a place in the menu that driver can set or select the size/ratio of the tyres in use? With the tyre size/ratio entered, will the car calculate and show the correct speed?

How to make the digital speed reading to be shown somewhere? (sorry if someone maybe have asked this question before)
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      01-20-2013, 01:33 PM   #11
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Originally Posted by ob2ob2 View Post
Is there a place in the menu that driver can set or select the size/ratio of the tyres in use? With the tyre size/ratio entered, will the car calculate and show the correct speed?

How to make the digital speed reading to be shown somewhere? (sorry if someone maybe have asked this question before)
I don't think there is anywhere in the menu that allows you to select the size/ratio used. I think the speedo is factory calibrated to a particular size and it's this factory value that the rotational TPMS uses to work out if you have puncture. If you look at your car door you'll see all the recommended tyre sizes. Whack them into this calc:

http://www.etyres.co.uk/tyre-size-calculator.htm

They will prob all be within a % or so of each other, most sites only recommend +/- 2.5%. Using the E91 winters I would of been +3.16% difference, which is about 3 mph slower than the speedo reading.
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      01-21-2013, 05:52 AM   #12
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Having only run on Winter Tyres since having the F30 I can't confirm until I put the summers on. What I can say is in the 1500 miles I have done most of the time I have sat at 70 mph on the dial when on the motorway and I'm still overtaking most cars on the roads so I wouldnt say mine was over reading at all!
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      01-21-2013, 06:16 AM   #13
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"There were some other qualities about the car on winter tyres that were less desirable, true. I noticed, for instance, that the speedo had become wildly ambitious; at a true 70mph it was reading almost 80mph, which meant the fuel range indicator was similarly off piste. And the car’s traction control also become neurotic, killing the power at the merest whiff of throttle, even on bone dry roads. "

Hmmm - the speedo on normal wheels/tyres over reads in any event (in my experience, most cars are about 4-5mph optimistic at 70mph true), so only some of the excess is due to different wheels/tyres
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      01-22-2013, 09:42 AM   #14
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Quote:
Originally Posted by davyk31 View Post
What's the offset for F30/31 models?
18" wheel 400 is 34 front and 47 rear

19" Wheel 403 is 36 front and 47 rear
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