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Subscribed! I love your boat tail.
The Honda Insight could achieve 100mpg (US) from the factory with a little extra air in the tires. You could look for some of the design elements used in that car perhaps? I'm happy to take some pictures or describe those I'm aware of.
I changed the engine in mine from the thrifty 1.0L to a power-oriented 2.4, and the best economy I've seen since then has been around 65mpg cruising, under the same conditions the old engine was showing 100mpg - cruising at no greater than 50mph.
It seems the new engine isn't (much) less efficient at producing power, but that it has larger parasitic losses. For instance, it uses a lossy alternator instead of a highly efficient DC-DC converter, it has a bigger water pump, bigger oil pump, the serpentine belt looks to be a little over twice as large, another cylinder's worth of frictional parts, a second camshaft, etc. Because of these, it doesn't seem to scale down as well - there's a larger fixed amount of fuel consumed just running the engine to do the same amount of work.
Where the old engine could produce (let's say) 20hp, the new one uses around 33% more fuel. However, the new engine can produce ~65hp at close to the same efficiency with my current gearing. Meaning, with pulse and glide (accelerating for periods with 65hp then shutting the engine off and coasting for a bit, rinse and repeat) I expect I could get very near that same 100mpg. It's possible I could get close to 100mpg with the new engine if I had MUCH taller gearing (all of those lossy parts would be spinning proportionally slower) but it simply wasn't possible to fit gears large enough in the gearbox.
All of this is to say, you may be able to squeeze a little more out of your engine with taller gearing and a set of underdrive pulleys, to slow down those lossy accessories. It's not a bad place to look for gains after you've already picked the low hanging fruit from rolling resistance and aerodynamic drag.
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