Jack’s current car gets 10 mpg. He’s trading it in for a car that gets 20 mpg. Delta between old and new mpg: 10 mpg
Jill’s current car gets 30 mpg. She’s trading it in for a hybrid that gets 45 mpg. Delta between old and new mpg: 15 mpg.
So which scenario is better from an environmental perspective? It’s actually Jack’s. The problem here is that measuring in miles per gallon can be deceptive. Instead, use gallons per mile. Let’s use a few examples:
All four cars get 100 gallons of gas. Jack’s old car will go 1000 miles. His new car will go 2000 miles. That’s a 100 percent increase. Jill’s old car will go 3000 miles. Her new car will go 4500 miles. That’s just a 50 percent increase.
All four cars have to travel 100 miles. How much gas will that require? Jack’s old car requires 10 gallons. His new car requires 5 gallons. That’s a 5 gallon improvement. Jill’s old car requires 3.33 gallons. Her new car requires 2.22 gallons. That’s just a 1.11 gallon improvement.
So it really is more important to swap out those Hummer H2s for Ford Explorers, than it is to swap out those Toyota Camrys for hybrid versions.