Converting to l/100km, 70mpg = 3.36l/100km, and 35mpg = 6.72l/100km, a difference of 3.36l/100km. On the other end, going from 6.72l/100km to 10.08l/100km is 23.3mpg. So while a 35mpg improvement from 35 to 70 would save you 3.36l over the trip, an 11.7mpg improvement from 23.3 to 35 would save you the same 3.36l. Going from 70mpg to 140mpg saves you just half that, 1.68l. Someone going from 15mpg to just 20mpg saves 3.92l over that trip!
I recently switched from a 30mpg Honda Accord to a 45mpg VW Golf TDI, but if I took the next step to a 60mpg hybrid, I'd only be saving half the amount in fuel (less actually, since diesel is about 5-10% cheaper where I am at the moment). While it's great to highlight the likes of the Prius, Civic Hybrid, etc., you really need to focus on the elephants in the room, those in the under 25mpg range that could easily switch to something close to 35mpg without having to pay the hybrid/electric/diesel premium.