Into the weeds engine design for fuel efficiency discussion.

Bewy said

Nope, smaller cu in doesn't necessarily mean better economy. Every engine has a 'sweet spot',
where all it's parts work together to give best mileage or best power, & rarely both....
A small engine has to lug. A big one doesn't.

It is very difficult to give an accurate answer to the original question because of so many variables, other than use a carb with smaller pri bores [ eg, a TQ ] & a high velocity dual plane intake. Good ign system like HEI that will fire a 060 plug gap.

I see a lot about piston crevice volume [ CV ]. Some people even use it in their CR calculations! Need to think why there is a CV: the top of the piston runs hotter where the rings are because it is closer to the combustion heat. If the piston diam was not reduced in this area [ CV ], it would seize as it heats up. The CV is all but gone at operating temp.

Click to expand...

Show me an instance where a larger engine gets better mpg for a road going vehicle.

Larger engines create excess power which is a waste in every regime except acceleration and ascent. Longer strokes, bigger bores, and large cam profiles all waste energy in order to gain power outside of the cruise regime, but average mpg is dominated by cruise.

When cruise mpg is the goal, minimizing ci is the lowest hanging fruit.

OP says he wants to tow - but does tow mpg matter as much as unladen? Without those details it's impossible to figure out which end of the displacement spectrum to be on. Knowing a max grade, weight, and operating elevation would make figuring out the minimum requirements a lot easier.

Also, typically a smaller engine with a turbo will out power and out mpg the larger na engine in almost every case. It can maximize torque when needed and use fuel sparingly when it's not. It's no secret why oems have had a love affair with boost in ecino cars for some decades.