From the 1920s to the 1970s, the evolution of engines (measured by compression ratio) and the evolution of fuels (measured by octane rating) occurred in tandem. Gasoline octane improvement during that period was likely due to refinery technology improvement and the addition of lead, which guards against engine knocking.

In 1973, the Environmental Protection Agency (EPA) mandated a reduction in the lead content of gasoline and eventually banned the use of lead in fuel for on-road vehicles. Since that time, oxygenates have replaced lead as anti-knock additives and the average octane rating of gasoline has been fairly constant at about 88-90 AKI (anti-knock index).

The engine compression ratio of new cars and light trucks increased along a similar course as octane rating from the 1920s to the 1970s. After that time, the average compression ratio continued to increase due to advanced engine design and controls, diverging from the octane trend. Higher octane levels can enable higher compression ratios, which would further improve engine efficiency.

Graph showing how engine compression ratio continued to increase after the 1970s, after fuel octane started to decrease. AKI = anti-knock index. Average octane rating based on refiner sales volumes and is based on a calendar year. Average compression ratio is not sales-weighted and is based on a model year. Source: DOE VTO.