35 MPG Duster

In the mid 70's I was able to get consistent 25 mpg with a /6 and 3 on the tree 66 Dodge Coronet. I advanced the timing some, added CDI ignition, and replaced the air cleaner top with a piece of Plexiglas.

I also did the same with a 273 2-barrel 66 Barracuda with auto trans without modifications.

I think the modern gas with ethanol might be about a 8-10% penalty. Modern oils and tires may help offset the weak fuel.

If you use a programmable EFI use multi-port, not TBI. TBI may only be sightly better than a carb. Good ignition and the ability to tune it (RPM and MAP) is also important.

High compression may not be as efficient as a lower compression engine at light load conditions.