Most people have no idea, but the entire speed limit is a scam simply to raise money. On January 2, 1974, effective January 6, 1974, during the whole OPEC oil crisis, to conserve gasoline during the 1973 oil embargo and resulting energy crisis, Congress came up with the nonsense of reducing the speed limit to reduce fuel consumption. They repealed the federal law in 1995, returning speed limit authority to the individual states. They did not change in most cases because they would collect fewer fines.
If you drive for 5 hours at 55 mph, consuming 30 mpg, and then at 75 mph, you get there in 3.6 hours at 24mpg, you used 11.46 gallons compared to 9.17 for the same distance, but you saved about 1.5 hours.
Nevertheless, nobody cares about safety. The idea that you are travelling at 65 instead of 55 has nothing to do with safety. It is all about money.