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Old 03-10-2013, 02:26 AM   #1
PanVet
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2008 Astra XE
2004 VUE 3.6L
Default New wheels

So the time has come to replace the factory 15 inch wheels with some 17s.

I have been told that it will change the actual speed reading of the car and I will actually be going slower then what the speedometer reads.

Any idea how much of a difference 15s to 17s will make?
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Old 03-10-2013, 11:48 AM   #2
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Default Re: New wheels

Quote:
Originally Posted by PanVet View Post
So the time has come to replace the factory 15 inch wheels with some 17s.

I have been told that it will change the actual speed reading of the car and I will actually be going slower then what the speedometer reads.

Any idea how much of a difference 15s to 17s will make?
The diameter of the rim itself doesn't make a difference, unless you want to donk your Astra (not sure if that's a verb or not). What matters is the overall outside diameter (OD) of the tread of the tire. You can use an online calculator such as this one to judge the difference. All you do is enter your existing tire size, and then check below to see how those stats compare to various other tire sizes. You can set the percentage below to widen or narrow the range of results (compared to your original tire OD), and see the difference at a specific vehicle speed.


PS: If you want to jump up to 17" wheels/tires, you should know that the Astra tire size found on the factory 17" wheels is 225/45R17, and they would be an exact match to the 195/65R15 tires that I believe you are running.

PPS: The speedometers of nearly all cars tend to read higher than your actual measured vehicle speed when calculated by GPS or covering known distances (with BMWs apparently being the worst), so you can err on the "slightly larger than stock" side and still be reasonably on target. Just not too big, since large jumps can start to cause rubbing issues inside the wheel well on the plastic fender liner or on some suspension components. I can't tell you how much is too much, because it varies from car to car (as well as how you load the car up with weight, how aggressive you corner, etc.).
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Old 03-10-2013, 12:17 PM   #3
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Default Re: New wheels

Fyi, the factory wheels were 16, 17, or 18 inch. Nothing smaller fits over the upgraded brakes we got in NA. You probably have 16 inch wheels, not 15's. As was said already, the outside diameter of the tires don't change between the three rim sizes.
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Old 03-10-2013, 03:23 PM   #4
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Default Re: New wheels

the outside diameter of the tire on the astra is the same as BMW and Porsche. 25". I think the factory 18s came with a tire that was slightly different than 25", or was narrower than the 225s that came on the 17" rims in order to not rub and be a 25" outside diameter.
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Old 03-11-2013, 09:53 AM   #5
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Default Re: New wheels

They are 16. I checked the label in the drivers door well.
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Old 03-11-2013, 11:39 PM   #6
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Default Re: New wheels

Hopefully this will be my last stupid question.

The tires measure 17x7.5 and the rims are the same measurement so I am assuming I can mount them together?
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Old 03-12-2013, 08:06 AM   #7
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Default Re: New wheels

The stock tire sizes are:

205/50/16 for 16" wheels
225/45/17 for 17" wheels (at least mine were)
225/40/18 for 18" wheels

Stick with those dimensions when you're shopping around for tires and the spedometer will be unaffected. I decided to go wide with my 18's and got 245/40/18 tires, which bumps up the total diameter about 0.7". The tire calculators said this would make my spedometer read a bit slow (actual speed 102km/h when the spedo says 100km/h), but I'm fine with that.
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Old 03-12-2013, 08:43 PM   #8
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Default Re: New wheels

I heard some people had a rubbing issue when going with 18s. Which was why I was going with 17s.
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Old 03-13-2013, 02:18 PM   #9
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Default Re: New wheels

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Originally Posted by PanVet View Post
I heard some people had a rubbing issue when going with 18s. Which was why I was going with 17s.
Shouldn't be a problem as long as the wheel has the same width and offset as the factory wheel, and you use the factory 225/40R18 size.
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