Great challenge! You should have agreed to neutral coasting even with engine on! It seems like a mighty challenge my friend! maybe you can barter with 0.9gallons on the downhill side? Test with GPS and act a fool.
add a massive cardboard/ducttape Aero shell past the back windows, per the template, add an aero nose similar to Aero Civic , remove all seats, doors and extra weight and replace with cardboard, Inflate the tires to 80psi, remove the back brakes except the hand brake, experiment with cylinder deactivation and get a MPGuino asap.