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Old 31-10-2005, 05:23 PM   #1
ENVI XR6
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Default 18 inch wheels and speedo

Hey just wanted to know if i put 18inch wheels on my au if it is going to mess around with the speedo?

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Old 31-10-2005, 05:50 PM   #2
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depends on your how you change the profile of your tyre.

Example: You currently have 225's at 60 profile. That means the sidewall is 135mm high. On 16" alloys, this means that you have (16"*25.4)/2+135mm = radius of your wheel. Means the radius is 338.2mm.

Going to 18" wheels... you want to keep your radius at say 338.2mm to remain 'accurate'. Just work the maths backwards.

(18"*25.4)/2+x=338.2mm

Solve for x:

228.6+x=338.2mm
x=338.2-228.6
x=109.6mm in sidewall size.

Say you are going for 18"x8" alloys, you might be looking at 245mm wide tyres.
So you have 245mm*Y=109.6
Solve for Y

Y=109.6/245
Y=44.74

So you can safely go for 45 profile tyres with 245 wide tyres on a 18x8 rim and maintain your speedo accuracy within a few decimal places.

Not too bad eh?
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Old 31-10-2005, 10:14 PM   #3
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im impressed my dad is good at maths so i will get him to work it out thanks mate
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Old 31-10-2005, 10:23 PM   #4
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Tell me what your current wheels are and i'll let you know
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Old 31-10-2005, 10:34 PM   #5
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they are just the stock au cheese cutters is has on them 205/65R15 94H
i hope thats wat ya need
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Old 31-10-2005, 10:48 PM   #6
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well 205 at 65 profile is 133.25mm

(15*25.4)/2+133.25 = radius
Therefore your radius is currently: 323.75mm

So to go to 18x8 alloys, you'd prolly want something like 40 profile on 235's (at a guess, but less find out on 235mm wide tyres.

(18*25.4)/2+Y= 323.75mm where Y = profile height.
Y=95.15mm
So using a 235mm wide tyre, with a 95.15mm height means it is a tyre 40.49 profile (close enough to 40).

So you would get 235/40R18 tyres.

Or guessing if you are going 245's you'd prolly go 35's.
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Old 31-10-2005, 10:53 PM   #7
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Quote:
Originally Posted by parawolf
well 205 at 65 profile is 133.25mm

(15*25.4)/2+133.25 = radius
Therefore your radius is currently: 323.75mm

So to go to 18x8 alloys, you'd prolly want something like 40 profile on 235's (at a guess, but less find out on 235mm wide tyres.

(18*25.4)/2+Y= 323.75mm where Y = profile height.
Y=95.15mm
So using a 235mm wide tyre, with a 95.15mm height means it is a tyre 40.49 profile (close enough to 40).

So you would get 235/40R18 tyres.

Or guessing if you are going 245's you'd prolly go 35's.

my brain hurts......... :
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Old 31-10-2005, 11:21 PM   #8
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Ok so i need to get235/40R18 tyres. i didnt quite get the next bit:Or guessing if you are going 245's you'd prolly go 35's.

Quote:
Originally Posted by parawolf
well 205 at 65 profile is 133.25mm

(15*25.4)/2+133.25 = radius
Therefore your radius is currently: 323.75mm

So to go to 18x8 alloys, you'd prolly want something like 40 profile on 235's (at a guess, but less find out on 235mm wide tyres.

(18*25.4)/2+Y= 323.75mm where Y = profile height.
Y=95.15mm
So using a 235mm wide tyre, with a 95.15mm height means it is a tyre 40.49 profile (close enough to 40).

So you would get 235/40R18 tyres.

Or guessing if you are going 245's you'd prolly go 35's.
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Old 31-10-2005, 11:35 PM   #9
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Ookaay.. _

In simple terms, 18X8 rim on a AU with either 235/40/18 or 245/40/18 will not "mess around with the speedo" that much. Do NOT go for 35 profile ( or 45 for that matter ) if you go for 245's.
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Old 31-10-2005, 11:38 PM   #10
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I'm sick of posting this site up. Bookmark it peoples. :P
(half way down)
http://www.fordmods.com/forums/documents.php?doc=30
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Old 01-11-2005, 10:21 AM   #11
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Decided what I had to say wasn't that important - and more a result of cuts in government education standards : :
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Last edited by parawolf; 01-11-2005 at 10:36 AM.
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Old 01-11-2005, 07:19 PM   #12
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245X40X18 will give you a rolling diameter 2 millimetres difference from your old wheels so they will not make any difference to your speedo.
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Old 01-11-2005, 07:48 PM   #13
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Quote:
Originally Posted by parawolf
well 205 at 65 profile is 133.25mm

(15*25.4)/2+133.25 = radius
Therefore your radius is currently: 323.75mm

So to go to 18x8 alloys, you'd prolly want something like 40 profile on 235's (at a guess, but less find out on 235mm wide tyres.

(18*25.4)/2+Y= 323.75mm where Y = profile height.
Y=95.15mm
So using a 235mm wide tyre, with a 95.15mm height means it is a tyre 40.49 profile (close enough to 40).

So you would get 235/40R18 tyres.

Or guessing if you are going 245's you'd prolly go 35's.
this is what i have on my AU and the speedo is the same as it was with the 16'`s on
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