What about propane? It was first interduced to the automobile in 1917. It never took off and less than 2% of the automobiles on the US highways today use it.
On a per unit weight basis, liquid propane contains about five percent more energy than gasoline; however, the density is 30 percent less. The net result is that a litre of propane (lighter) contains about 28 percent less energy than a litre of gasoline. Propane has an octane of 110. Propane has been used in racing and hi-performance cars but never been popular.
What about hydrogen?