worthwhile putting a 2.0 into a 3500 lb '85 Monte Carlo?

[MetroMPG]

Received this question at metrompg.com...

I would like to know what possibile mpg goal there is for putting a very small engine in a larger car. Even if you only get 35mpg, that is still better than any 3500lb car is getting now. I want to know if it is even worth the trouble to put a 2.0L (85hp) engine in a 1985 Monte Carlo.

The writer originally wanted to know if I'd test my car by adding a lot of weight to see what happens to the fuel consumption - as a point of reference.

I would say the Prius is an example of a "small" engine in a larger car, and it gets great mileage on the highway, even when the EV systems aren't contributing.

Of course it's a wee bit more aerodynamic than an '85 Monte Carlo...

And the Prius is an Atkinson ICE...

Any other examples?

I read somewhere that in some east Asian markets, Mercedes sells versions of its S Class with a 2.0L due to local regulations.