[aerohead]

Originally Posted by BabyDiesel

With more weight reduction, LRR tires, a Cd of 0.19, a drivetrain efficiency of .90 for a manual swap, and 77* air puts me at 70-80 mpg for my usual drive speeds of 45-50 mph (theoretically)

Aerodynamic & rolling resistance, power & MPG calculator - EcoModder.com

The Cd 0.19 GM Ultralite was rated at 100-mpg @ 50-mph.You're frontal area wouldn't be much different,and at cruising speed you're weight wouldn't slay you.
GM did have tires with something like 60% less rolling drag than conventional steel radials of the day.I suspect that this gap has closed since 1991.