Even with the world's largest aviation gasoline production, the U.S. military still found that more was needed. Throughout the duration of the war, aviation gasoline supply was always behind requirements and this impacted training and operations. The reason for this shortage developed before the war even began. The free market did not support the expense of producing 100-octane aviation fuel in large volume, especially during the Great Depression. Iso-octane in the early development stage cost $30 a gallon and even by 1934, it was still $2 a gallon compared to $0.18 for motor gasoline when the Army decided to experiment with 100-octane for its combat aircraft. Though only 3 percent of U.S. combat aircraft in 1935 could take full advantage of the higher octane due to low compression ratios, the Army saw the need for increasing performance warranted the expense and purchased 100,000 gallons. By 1937 the Army established 100-octane as the standard fuel for combat aircraft and by 1939 production was only 20,000 barrels a day. In effect, the U.S. military was the only market for 100-octane aviation gasoline and as war broke out in Europe this created a supply problem that persisted throughout the duration.