Suburban vs. Impala (or similar) fuel economy?

[66Submarine]

Been thinking about this recently; I have the HD/WTF truck thing well covered with my one ton truck(s), so another thing I'd be driving really wouldn't necessarily need to do that.

So, I've been considering something like an old four door Impala vs. a Suburban; certainly seems like the car would be a lot more aerodynamic and get much better fuel economy rolling down the highway in the hammer lane than the Suburban would.

I know most of the time something like this comes up some guy with a sunk carb float likes to talk about how every car/truck from the 60's gets 5MPG or something stupid, but does anyone have any real input? Seems like the immensely sleeker car would have to net me some noticeable gains over the much more brick-like trucks/'burbs.