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Does anybody know exactly the equivalence between the extent of offset, the width of rim and the thickness of spacers?
For example: if I have 7J rims ET 25 on the front and want to put 7,5J on the rear, which ET should they have to mantain the same ratio of the front ones?
Another question: using 7J rims ET 20 corresponds, as a result, to use spacer how much thick? And 7,5? And 8J?
I don't know if I managed to explain me.... maybe not.
I hope you will forgive me... :D
 

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Because no-one else has answered... :p

If you understand offset properly, you'll be able to answer your own questions.



If you now compare 7J ET25 with 7.5J ET25, the 7.5" wheel is 0.5"/~12mm wider. It will 'sit' 6mm more outwards and 6mm more inwards.

If you want the outside edge to be in the same position, you would need 7J ET25 and 7.5J ET37/38.
If you want the inside edge to be in the same position, you would need 7J ET25 and 7.5J ET12/13.

:)
 

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Does anybody know exactly the equivalence between the extent of offset, the width of rim and the thickness of spacers?
Asking questions like this on a forum will likely get you a lot of "spazzatura". But, if you must...

You're missing one more critical measurement - backspacing.

-As you can see from the diagrams shiney_car posted, ET is measured from centerline of wheel.
-Spacers do not affect ET of a wheel, they affect the backspacing of that wheel.

There are some good calculators out there (if you had spent 2 minutes to look).
Tire Size Calculator - tire & wheel plus sizing

Wheel Offset Calculator

I would add one more thing. Since you Europeans like to stretch 205's over an 8" rim, the diameter calculator will not be totally accurate. The overall diameter will be shorter than the calculator says due to the inward angle of the sidewall.

So, to answer your question:
if I have 7J rims ET 25 on the front and want to put 7,5J on the rear, which ET should they have to mantain the same ratio of the front ones?
-Don't even need a calculator for that. Since ET is measured from centerline, a 1/2" wider wheel will add 1/4" to both sides (or 6.35mm). So, the 7.5J rim would need 31-32mm ET to maintain the same backspacing of the 7J rim.
 
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