I had 69 and 71 C20, 350 engines. Tuned with vacuum gage and by ear I was able to get 18MPG fully loaded. BY that I mean exceeding load capacity by almost 150% and towing heavy.
Same tune up method, Dodge 318 Commander motor home pulling a Forde F150 4X4, 18 mpg. The 4X4 itself could not get 13 ever.
Then I had an 81-82 chevy 6 cylinder (don't remember engine) truck. Supposed to sip the gas at something like 24mpg. I got 11 mpg, and could never get better hi-way or city, literally same mileage always.
Drove a RENTED 2011 Dodge Challenger, fully loaded trunk and 5 passengers. 600 MILES at 75-80 mph average, 36mpg SF to Seattle through the Sierra and cascades...twice. I was tempted to purchase that vehicle from the rental agency. But it was during 911 and I had lost my job.
Rented a 2011 Prius in CA in 2019 with 150K miles on it (private rental), drove Los Gatos CA to Carson NV, then on and around and back always at posted speed. 53 MPG. HUGE mountains and great mileage.
It is NOT a crap shoot it is insanity. The numbers and tune up information etc mean NOTHING. For some reason I was able to use vacuum and HEAR the engine run better than the specs (before I lost my hearing). And occasionally an engine/power system produces Far better than any average.
Not sure what it takes today but it is time that engines begin to get guaranteed mileage or returned!!! However there is not much excuse for a truck with a smaller engine not being able to see 30+ MPG loaded. Consider either the motor home OR the Challenger...either one has so much more HP and one has 3X the wind/drag of a truck by itself.
IT SEEMS TO ME THAT ALL MANUFACTURERS NEED TO FIND THIER SOLD VEHICLES THAT GIVE EXCEPTIONAL MILEAGE AND PURCHASE THEM BACK TO EVALUATE AND FIND OUT WHY AND REPLICATE THE RESULT. THIS MIGHT BE THE KEY TO ACTUAL DECENT MILEAGE IN ALL VEHICLES!!!!