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?
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.