Why is a barrel of oil 42 gallons instead of 55 gallons?

A barrel of oil is 42 gallons instead of 55 gallons due to historical reasons dating back to 1860s Pennsylvania oil boom. In early oil trade, oil was shipped in various wooden barrels of different sizes, often 40 to 50 gallons, causing confusion and cheating. In 1866, Pennsylvania producers agreed on standard 42 gallon barrel, based on 42 gallon tierce, an old English wine barrel size, plus extra 2 gallons allowance for evaporation and leakage during transport, so they used 42 gallons to ensure buyer still got 40 gallons net. This 42 gallon standard was adopted by Petroleum Producers Association and later by US government. The 55 gallon drum commonly seen today is later steel drum introduced in early 1900s for shipping oil products, but crude oil measurement remained 42 gallons for historical and legal reasons. So 42 gallon barrel is traditional oil industry unit, not physical barrel used today, oil is now shipped by pipeline and tanker, but pricing still per 42 gallon barrel. So reason is historical standardization in 1860s to prevent fraud and account for loss.