It's not about how much gas you save.
It's about how much - or how little - you use
Not %-wise, but in gallons / liters.
The same % improvement will translate into many more gallons / liters not used on a gas guzzler, than it will on an already fuel efficient vehicle.
Stated otherwise, it becomes more difficult and less (financially) rewarding to improve upon an already fuel efficient vehicle .
That's the law of diminishing benefits.