the catalytic converter was the death knell for pickup gas mileage. I think '74 in California and '75 everywhere else.
I was doing odd jobs for an RV sales and repair place in California. Part of it was running out for new inventory like pick up campers and travel trailers.
I would do the run from San Bernadino to one of the manufacturers in OC, Empty down and 11 or 12 foot camper loaded on the way back.
With the '73 chevy 350 auto shop truck I would fill up the 22 gallon single tank just as I left and have a couple of gallons left when I got back.
With the brand new'74 chevy 350 auto shop truck with that new fangled catalytic converter thingy. I would fill up both tanks (I think 20 and 22 gallon) and barely made it back with both tanks on fumes. Dealer couldn't find anything wrong with it either--said they were all like that.
Never could understand how you could double fuel useage to make the exhaust cleaner.