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Australian Falcons Discuss the australian born and bred models here. Includes the 80's 90's and present day Falcons offered by Ford Oz.

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Old 03-20-2006, 23:23   #1 (permalink)
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Rolling diameter?

Hi there
I would like to lower my BA XR6 with king SSL but my question relates to rolling diameter as i want to try to retain a much front spoiler clearance as possible. I currently have the standard BA XR6 235/45/17 but would like to upsize to either BA XR8 245/40/18 but 1st choice would be GT-P Wheels 245/35/19
Does any1 know if the 18s or 19s would be the same rolling diameter as my current package? Im hoping the 19s would give me a little more clearance than if i put SSLs in with current wheels. Advice reqd.
thanks
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Old 03-20-2006, 23:44   #2 (permalink)
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Re: Rolling diameter?

changing the rolling diam of the wheels and tyres will stuff around with your speedo.

You can calculate the rolling diam. the side profile of the tyres is as follows

245/40/18's... is 40% of 245 is the side wall profile in mm.

therefore the diam is 18*2.5 + 2 * .4 * 24.5

this is the two sidewalls top and bottom plus the wheel. and you don't want to change it unless you don't mind the speedo being out....

you can calculate the differnce in speedo by calculating the difference in circumference of the tyres.

i've got 18's and i tried to match the rolling diam as much as i could to the stock equipment.
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Old 03-21-2006, 00:09   #3 (permalink)
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Re: Rolling diameter?

Most tyre manufacturer's websites show overall diameters and revolutions per KM (or mile). I read on here that your speedo recalibrates itself somehow???
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Old 03-21-2006, 00:09   #4 (permalink)
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Re: Rolling diameter?

Try this tyre size calculator. Scroll down to "why would i want to change my tyres and rims anyway" , this will give you exact speed difference between sizes and profiles.

Note: American site, speeds in MPH.

http://www.carbibles.com/tyre_bible.html
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Old 03-21-2006, 00:28   #5 (permalink)
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Re: Rolling diameter?

Quote:
Originally Posted by AU2XR85.0
Try this tyre size calculator. Scroll down to "why would i want to change my tyres and rims anyway" , this will give you exact speed difference between sizes and profiles.

Note: American site, speeds in MPH.

http://www.carbibles.com/tyre_bible.html
Thanks for your reply's everyone

I like the feature that reclaculates circumference in mm & speed in MPH

it appears about an 11mm increase if i went for the 19s im lookin at.
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