Speedo readings when changing wheel size

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Metroreg
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Speedo readings when changing wheel size

Post by Metroreg » 05 Dec 2017, 20:01

Just a question well could be a stupid question, as some of you know I'm changing the size of my rims and tires on the 330

so had 18" on the car now change to 19" rear 235/35/19 on front 19" 225/35/19 will this change the speedo speed or not :-ss
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Re: Speedo readings when changing wheel size

Post by cockneyrebel » 05 Dec 2017, 21:12

Metroreg wrote:Just a question well could be a stupid question, as some of you know I'm changing the size of my rims and tires on the 330

so had 18" on the car now change to 19" rear 235/35/19 on front 19" 225/35/19 will this change the speedo speed or not :-ss
It'll read what the sensor reads I think you'll find Sam

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Re: Speedo readings when changing wheel size

Post by RayB » 05 Dec 2017, 21:15

The rolling road diameter needs to remain the same. So if you put 14" rims on you need massive tyres. If you put 20" rims on you need an extremely low profile.

Just put your existing and new into will they fit and try to keep the percentage difference as low as you can

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Re: Speedo readings when changing wheel size

Post by Metroreg » 05 Dec 2017, 21:24

Thanks To you both just checked on the net, it would be a slower speed, glad i checked just in case i was going a little faster
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Re: Speedo readings when changing wheel size

Post by M60NJP » 06 Dec 2017, 07:29

With 10% + 2 on most speeding prosecutions, you'd have to make a major change to be 'at risk'!!

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Re: Speedo readings when changing wheel size

Post by Lewy » 06 Dec 2017, 21:25

I'm sure you've been here before - http://www.willtheyfit.com/" onclick="window.open(this.href);return false;

You'll need to get as close as you can as your ASC sensor will kick in if it thinks your rears are going quicker than your fronts. The guidance is 2.5% but I've been less than that but above 2% and my ASC kicked in when it shouldn't have done.

Whilst your question is about speedo reading, to avoid ASC issues, you want to be:

a) Less than 2% out anyway (good practice) and
b) The same 'out' on the fronts as on the rear

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