First..some basics ..as I ..see them
diesel will ALLWAYS get better mpg than gas..Ask a trucker.
Turbo will help any diesel LOTS..( been there , done that)
Water injection helps air density, keeps a turbo cooler.
Start ..with a Three cyl. Turbo diesel , with water injection.
next..old school semis had " two speed rear ends" which ..in theory could give
the effect of "double overdrive" ..Squeezing the Horsepower out of the torque.
.so to speak.
Recall that a 55 nash metro ..would get great mileage, was a square brick, no plastic, or aluminum even, no fuel injection, no computer. If one swapped a diesel ..gain of 20-30 percent.
If aerodynamic..Huge
gain...
LRR tires..more gain still..
I feel it is possible...if approached in this manner. !
KEEP AT IT!!!