In economics, everything is on the margin. In engineering, constraints tend to define tradeoffs. If we look at the population, then for everyone that needs/wa

In economics, everything is on the margin. In engineering, constraints tend to define tradeoffs. If we look at the population, then for everyone that needs/wants a car of size X, age Y, and cost Z, then fuel efficiency regulations that increase the engineered complexity of the vehicle will push up the cost and mean the consumer has to trade off with a smaller car, older car, or some other aspect. Discretizing the population into poor people that can't afford a truck and rich people that can is meaningless when there is an entire distribution of people's incomes and spending preferences.