It's common sense really, a full tank of gas each time you go to the pump (if you can afford it) saves you time and money. Each additional trip to the gas station is more fuel burned and during times of short supply, you are less likely to be part of that long queue of vehicles.
Further, each time the fuel cap is removed or the car engine is turned on , there is some element of wear and tear involved. All these are reduced by fewer stops at the pump.
Keeping tire pressures at the recommended levels is a fuel saving tip taken for granted. The extra drag required to propel the vehicle when tires are not at optimal pressure burns more fuel.