[Big time]

IMO the biggest factor affecting mileage is vehicle weight.
Unless you use exotic materials a lighter large-sized vehicle will tend to be fragile.

Thus we need rollcage style rigidity around the cabin. No problem here so far as race homologated rollcages only weight a few hundred pounds, yet they withstand spectacular accidents.

The problem comes in crumple zones.
Even if optimally shaped sheet metal is a poor material for crumple zones: not elastic and heavy.

This is where foam metal will come to play.
They offer exceptional shock absorbance for its light weight and small volume.
For example you can google titanium and aluminum foam crumple zone reasearch.

Thus IMO the mpg futures is in foam metal crumple zones.