Typically in the US, gasoline for cars is a mixture of refined crude oil and some ethanol up to 10%.
Since ethanol in the US is corn based, by buying gasoline are you buying kitniyot?
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Dose of Halacha goes through the various opinions of toiletries that contain alcohol over Pesach. He explains why it isn't a problem.
R’ Ovadia Yosef (Yalkut Yosef 447:2) permits using products that contain denatured alcohol on Pesach. R’ Chaim Elazar Shapira (Minchas Elazar 5:37) writes that while people are particular, once alcohol has been denatured, it is not considered rauy l’achilah. Thus one may drive a car (on Erev Yom Tov and Chol Hamoed) even if the petrol is mixed with denatured alcohol.