I spent ~2k on upgrades without increasing displacement and the gain was marginal. e-fan was the biggest gain.
Better fuel economy is proper tire inflation.
Go no faster than 65 mph
Keep vehicle in good running condition.
Correct speedo gear for tires and gear ratio
Avoid gas with ethanol.
Know your tank size. Mine is 19 and normally fit 18.3 in. If I go over 19 the pump is off and I tell them about it.
Biggest is calculating mpg correctly. Full to empty tank. Divide miles driven by gas pumped to fully fill tank.