[photochemsyn]

This is probably a fudged statistic, because you have to add oxygenates to what's called 'pure gasoline' to get it to burn cleanly and efficiently, which bulks out the volume considerably. (These kinds of arguments were also trotted out in favor of tetra-ethyl-lead to provide the 'oxygenate' which only needs to be added at 1% or IIRC, compared to 10% ethanol blend or similar usually).

Fuel blending for optimal combustion characteristics is a complicated business, most of what people claim about this issue is not very reliable.