The subaru loses about 10% due to the 4wd ... so leave subaru out of
the identical cars. The 5-speed ... 5th is typically an overdrive and
wins hands down on the highway ... or, if you don't do much
highway/freeway driving, not much point. My '96 4.0L jeep Cherokee at
about 3100 lbs is a 4 spd auto (overdrive) with lock up torque
converter ... and consumes 10 litres/100km at a steady (cruise control
helps the mileage) 100 kph hour at about 1800 rpm ... and that's about
the same mileage as my subaru got ... at 3000 rpm. Driving in the city
I normally don't exceed 1800 rpm in any gear and the acceleration is
fine by me. There's always the guys in their little 4-cyls winding it
out between traffic lights ... sure it's fun ... but they don't need to
develop that full horsepower at high revs to move those little cars
from one light to the next. Got to admit though ... if I still had the
stick, I'd be winding it out too, and downshifting to brake.

Back to your post ... imo ... with todays engineering, and adding to
your equation equally skilled drivers ... mileage would be about the
same.