17"&18" rims and effects?

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hi guys,
Can you please tell me how your aftermarket rims affected your performance and fuel consumption? before and after range per full tank might be an accurate reference

there is a comon belief here which says the consumption rises even if you choose a lighter alloy rim than the stock one..

you might not care how much you consume in US,but its like a nightmare here in europe although we have diesels :)

thanks in advance