Well, the example I can come up with is the 2010 Escape Hybrid fwd and the 2010 Fusion Hybrid fwd. The both have the EXACT same powertrain. The Fusion is rated for 41 City 36 Highway 39 Combined, the Escape is 34 City 31 Highway 32 Combined. That's basically a 7 MPG difference in the combined rating.
Thanks all, the trip is almost exclusively highway. I know it is hard to figure out, but thanks for the examples. Both cars don't really weight the same thing, but I was just trying to simplify things a little. The Galant has been gutted for weight savings so in actuality it may be similar to the Talon in weight. It is not the same as comparing say an LT1 Camaro and an LT1 Impala SS. The weight difference is much closer to begin with between the 2 vehicles. The Galants weren't that large back then.
For a highway trip weight won't matter. If you're sure the entire drivetrain is identical (transmission ratios, final drive ratio, tire size) then you're looking at aerodynamic differences and probably some rolling resistance differences too.
Also don't forget that aerodynamic drag is not just the coefficient, but the coefficient multiplied by frontal area. Does frontal area differ much?
I don't see why. You store energy while ascending and release that energy while descending, there's no reason to lose anything. If anything it'd be an advantage, allowing you to P&G on the highway while keeping a steady speed.
Mountain area should almost always be worser for mpg, because when going down there's definitely corners where you need to brake/engine brake and you can not keep on 'gliding' downwards.
That's all loss of energy.
Can imagine it's a bit better on highways, but for safety you need to keep a bit of steady speed, no?