Practical difference between 17x7.5 and 17x8 wheels?

[OZGOD]

G'day everyone, I'm looking to upgrade my 2005 A4 1.8TQM wheels (which are the standard 215/55/16s) to 17" or 18"s. I was browsing Tirerack's selection and I notice that some of the wheels that are suitable for my A4 are 17x8 wheels while others are 17x7.5 (I'm guessing that the 2nd number is the rim width). Can anyone explain what the practical difference is in terms of driving and should I care whether I get 7.5s vs 8s?

I know that the rim width should be narrower than the width of the tire, but with 8s that converts to 203mm which is still narrower than most tires I will plan to run (in the 215-240 range).

Thanks for any help.

[ratty]

There's an optimum rim width range for each tyre size. Once you go outside of this range, people start telling you that you can't do certain things.

e.g. for a 235/45R17 tyre, the rim width "should" be 7.5 to 9 inches, for a 245/40R17 it "should" be 8 to 9.5 inches, for a 255/40R17 it "should" be 8.5 to 10 inches etc. If you look up data from tyre manufacturers, they will give you a range of rim widths for the tyre you want to fit.

So, the practical upside of this is that if you get a 7.5" width rim, you should fit a 235 or 225 width tyre, if you get an 8" width you can go wider.

Of course, I am running 255 on a 7.5" rim & they are fine, but as has been pointed out elsewhere, this combo may not work for everyone & will be dependent on the tyre construction.

[OZGOD]

There's an optimum rim width range for each tyre size. Once you go outside of this range, people start telling you that you can't do certain things.

e.g. for a 235/45R17 tyre, the rim width "should" be 7.5 to 9 inches, for a 245/40R17 it "should" be 8 to 9.5 inches, for a 255/40R17 it "should" be 8.5 to 10 inches etc. If you look up data from tyre manufacturers, they will give you a range of rim widths for the tyre you want to fit.

So, the practical upside of this is that if you get a 7.5" width rim, you should fit a 235 or 225 width tyre, if you get an 8" width you can go wider.

Of course, I am running 255 on a 7.5" rim & they are fine, but as has been pointed out elsewhere, this combo may not work for everyone & will be dependent on the tyre construction.

Thanks mate. So as long as I stick within this "optimum range" I should be right?

Also, I noticed that different wheels have different offsets. How do I know what my "stock" offset is?

Also, are you from Sydney, OZ? I moved to Boston from Sydders a couple of years ago - missing it badly!

[98a4]

You seem 100% new to this, if you were on tirerack check their tech section it i will answer all of these questions.

Lower et(offset) = sticks out towards or past the fender more. (poke)

for a 7.5 or 8" a 225 is standard.

[ratty]

Also, are you from Sydney, OZ? I moved to Boston from Sydders a couple of years ago - missing it badly!

Yup, that's the one. Bloody 8 degrees this morning though I hate winter...

Stock offset is +45 on the 17x7.5's. Check out carbibles.com - there's a calculator on there IIRC that shows what the offsets really mean & what you'll need if you go for a wider wheel. Anything from +35 to +45 should be safe provided the tyres aren't too wide.

[OZGOD]

Yup, that's the one. Bloody 8 degrees this morning though I hate winter...

Stock offset is +45 on the 17x7.5's. Check out carbibles.com - there's a calculator on there IIRC that shows what the offsets really mean & what you'll need if you go for a wider wheel. Anything from +35 to +45 should be safe provided the tyres aren't too wide.

Ah, the 8 degree winters - geez mate whenever it gets that warm in winter here it's cause for celebration. It was 8 degrees a few times in late spring here. It was also 8 degrees in winter a few times, 8 degrees Fahrenheit that is. Did I mention that I missed Sydney?

I did check out both the Tirerack info as well as Carbibles and I think I should be right with that offset. Cheers mate!

[ratty]

No worries. I thought you might like the 8 degrees bit: I've heard Boston ain't so hot. Down to 3 degrees this morning - snowflake for the first time this winter. Hasn't cracked 20 during the day for about a week & a half... I might have to go on holidays to Brisbane to warm up soon