Well, I suppose 25 mpg with a Nova 350 could be accomplished with the Economaster carb, an overdrive transmission and very careful driving. I know that one time during the gas crisis of 1979, a buddy and I were able to get 20 mpg out of his '69 Chevelle SS 396 on the highway between Minneapolis and Des Moines. We had to -- hardly any gas stations were open, since it was a Sunday. Also, the 1976 Plymouth "Feather Duster" was reputed to get up to 36 mpg on the highway -- and at only 180 pounds lighter than a conventional Duster, it was a fairly large car by today's standards.
I think mopar's real point is very valid, however. If respectable mileage was possible with what were, mechanically, relatively primitive vehicles, why isn't it a lot better today? Now, you have to get something like a Honda Civic to see figures like 36 mpg on the highway. And the tiny SmartCar only rates about 40 mpg. For the millions and millions of dollars that have been spent on research over the years, you'd think there would be a lot more to show for it.
Speaking of mileage, I should note that last month, my wife and I were back in Des Moines to see her relatives. We rented a Dodge Caliber, and it got 24 mpg in strictly in-town driving -- I didn't even use the freeway. Not a bad vehicle, except you can't see the front end at all because of the extremely long dashboard and the way the hood slopes downward -- and I'm 6-2. She's only 5-3 and didn't even want to try driving it because because of this rather strange feature.