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Old 18-02-2014, 09:15 AM   #1
shayne93
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Default Changing tyre size on XR8 - 17 inch.

My AU 2 XR8 (Auto - 3:45 LSD) has 235/45 17 tyres on it but the speedo is 4-5% slow ie. sit the speedo on 104-105, car is doing 100.

As most 75% of my driving is on the highway in 100 zones, it is quite frustrating.

I was thinking I could use a speedo corrector, ie. like the jaycar one, or change the speedo gear to one for a 3:23 ratio auto.

But... Running 235/50 or 245/45 tyres could fix the problem and give a slightly better ride.
I had 235/50 tyres on the other AU ute and the speedo was spot on, I have 245/45 on the back of that one now and the car does 98-99 when reading 100 on speedo, which I'm cool with...

What are your thoughts?

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Old 18-02-2014, 12:43 PM   #2
Uncle_Ken
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Default Re: Changing tyre size on XR8 - 17 inch.

I know everyone like a 'low look' but everyday drive I'd take the better ride path.
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Old 18-02-2014, 02:32 PM   #3
shayne93
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Default Re: Changing tyre size on XR8 - 17 inch.

We're talking 5%.. Height doesn't bother me. Thanks UK!
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Old 18-02-2014, 04:08 PM   #4
mif73
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Default Re: Changing tyre size on XR8 - 17 inch.

I had 235 45 r17 all round on 3.23:1 fairlane, the speedo was like yours (a few km out).

I ended up fitting 255 45 r17 (not such a common size) to fill the rear guards a bit (max legal width tyre for an 8 inch wide 17 inch diam rim).

The speedo is now spot on with the gps.

245/45 would be good, go for it.
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Old 19-02-2014, 12:13 AM   #5
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Default Re: Changing tyre size on XR8 - 17 inch.

245/45 is what is standard for commodores, I think, so would be cheaper than a 235/50, and would give the same overall diameter.
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Old 19-02-2014, 04:31 AM   #6
shayne93
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Default Re: Changing tyre size on XR8 - 17 inch.

Fg started with 235/50, changed to 245/45 in 2010/2011.
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Old 19-02-2014, 05:20 PM   #7
auIILTD
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Default Re: Changing tyre size on XR8 - 17 inch.

104 at 100 I have found to be pretty standard. So to bring the speedo down, you will need a larger rolling diameter.

This site might help.
http://tire-size-conversion.com/tyre-size-calculator/

Cheers
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Old 19-02-2014, 06:34 PM   #8
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Default Re: Changing tyre size on XR8 - 17 inch.

Quote:
Originally Posted by auIILTD View Post
104 at 100 I have found to be pretty standard. So to bring the speedo down, you will need a larger rolling diameter.

This site might help.
http://tire-size-conversion.com/tyre-size-calculator/

Cheers
Stu
Great site Stu
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Old 19-02-2014, 09:19 PM   #9
mif73
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Default Re: Changing tyre size on XR8 - 17 inch.

If you cant be bothered with metric to imperial conversion then you can try this one too:

http://www.willtheyfit.com

It also has a handy visual reference in regards to wheel widths and offsets.

Cheers.
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Wife unit's : 2010 update FG XR6, reverse cam, HR 2300kg Hitch, German Maestro speakers, parcel shelf sub, trimmed mudflaps, batman stickers.
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