235/40R17 1 2 3 1. Size in Millimeters 25.4mm equal 1 inch 2. Ratio of sidewall 235*.40=94mm 3. Rim DiameterSo if you take 235 and divide it by 25.4 you get 9.2" inches On 17s I think 1 to 1 1/2 inches smaller is acceptable this depends heavily on the sidewall ratio and tire brand but on 40 series and 45 series. Thus for a 235 I think you should get atleast an 8" Rim. However I have a 235/45R17 on a 7.5" rim and it works but probably would look a little bit better if was wider. However if you were thinking maybe a 255/40R17 then 255*.40 = 10 inches atleast an 8 1/2 rim. I hope this answers your question..

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