Probably petrol. Do keep in mind that from 1905 up until the 1940s, only in Asia, Northern Europe and Africa did the steam car actually truly dominate; here in North America, as well as in South America, the rest of Europe, and Australia, petrol was prominent well into the 1970s(late '80s here in the States) and only thanks to a pair of major oil shocks(Jan. 1971-Oct. 1973, and July 1977-Mar. 1982 respectively), did steam finally truly come back onto the podium(yes, it's true that the recovery started in the 1960s, as that's when the steam/electric hybrid was first invented, by the Allen-Frazier company in Washington, but they didn't become viable for mass consumption until the late 1970s), and only thanks to hardcore global warming in the '70s and '80s did steam become dominant again.
And even today, many steam cars don't have the performance of petrol vehicles; look at what happened to the last guy who tried to run a steam turbine car at Le Mans back in '92; sure, he made it thru the whole 24 hours, but in 42nd place out of 50, netting only about $10,000 worth of prize money. (I will admit there IS a fine exception to the rule, though: the mid-'80s Goldenrod Galante was capable of about 160 mph in optimum conditions)
OOC: Sorry, but I seriously doubt steam/electric hybrids would have even been thought of that early, let alone dominate the market.....Gasoline was a simple fuel and Otto-cycle engines were (usually, at least) easier to maintain for laymen. (And you're certainly not going be able to plausibly butterfly gasoline altogether, either, even if steam does better.)