This comparison was done a couple of pages back - 20 x 9.5 ET35 compared to Suzuka (20x8.5 ET38 ):
If you're looking at going 20 x 10 + 30 then this is 12.5mm (0.5") wider again and 5mm more aggressive in offset. You'll get inner clearance similar the the above 20 x 9.5, however you'll have 27.1mm more poke than the stock Suzuka wheel (assuming that's what you're running). You just need to get a measuring tape and and measure from your wheel to the arch to find out how much space you currently have and then choose a wheel width/offset that fills the gap.
If you have another stock wheel (other than Suzuka) which may be the case on other countries then you'll need to get the size of your wheel and put it into an online calculator - Will they Fit - The Wheel Fitment, Tyre Stretch, Rolling Radius and Speedometer Error Calculator
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