"Here's a little test:"

Which saves more gasoline, going from 10 to 20 mpg, or going from 33 to 50 mpg?

If you picked the second that's exactly backwards. Replacing a 10-mpg vehicle with one that gets 20 mpg saves five times the gasoline, replacing a 33-mpg vehicle with one that gets 50 does.

Here’s the math. replace your old (10 mpg) with a new (20 mpg), over 100 miles you cut your gasoline consumption from 10 gallons to 5. That saves you five gallons. If you swap your old (33 mpg) for a new (50 mpg), that only saves you one single gallon over the same distance.

I did not know that!