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Thanks for the theory behind the P&G on the diesel, makes sense to me.
I found a person with a similar truck as me, they have much higher geared axles though, 3.08 vs mine being either 3.55 or 3.73 (pretty sure it's 3.55 though). They have some areo mods (air dam, side skirts, and lowered 4in) and they pull ~26mpg. Based on what I've read on gear ratios and some experience, it's pretty 1:1 for the ratio difference, so 3.55 gearing is about 13.2% lower geared, so their mpg with my gearing should be around 22.57mpg, so I think an unmodded truck getting around 20mpg seems about spot on.
I never quite figured out why my corolla got such good mpg, I didn't do anything super crazy with it. When I was hitting near 45mpg tanks, it was 45mph back roads, ~1hr drive, and half engine bay belly pan and about 1/4 air block at the front of the car, max side wall pressure on tires and drive it for mpg (high load take off, cruse highest gear lowest rpm effectively). The car liked to go into full advanced timing, 39 degrees if I remember right, but if I could just barely touch the gas just right I could get it to pull back the timing to somewhere around 26 degrees and the mpg jumped way up. That car was a bit of an odd duck, it's sitting with 305k miles now, Michigan rust got to it, hoping to have it fixed for this winter.
Based on a couple sources, the effect of adding propane is similar to nitrous. Both help the diesel get burnt more completely and both added together gives a tiny bit of gain from the extra fuel (propane). The catcher is, nitrous only works basically at full throttle while propane hits a nice wide range of rpm.
I never really been into diesels much in the past, but it seems that they can really pull some impressive mpg numbers even in large vehicles. 20-22mpg Is kind of my target, if I can pull 25mpg+ I'd be quite impressed and probably be kicking myself for not doing the truck upgrade sooner lol.
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