PA28 pilot reported partial power loss followed by a total engine failure during cruise resulting in an off airport landing.

Narrative

Enroute to ZZZ from ZZZ1; the engine of the aircraft failed following a brief period of reduced power approximately 20 miles short of the destination. Care was taken to ensure that the tanks were full prior to the flight and that the fuel tanks were changed in accordance with the POH. The plane was landed in a field nearby; with no injuries nor damage to the aircraft or property on the ground. Fuel burn calculations indicate that at the time there should have been at least 2-4 gallons present in the main tanks of the aircraft; assuming the most conservative fuel burn; which were being used before and during the [situation] (the tank was switched from the right main to the left main with the fuel pump on; and every other checklist item being followed). The left main tank was working normally prior to being switched approximately an hour before; but switching to it in this instance did not provide an increase in power. The fuel gauges additionally indicated about 4 gallons in each main tank. I am unsure as to the cause of the failure; but I suspect a mechanical issue; possibly in the fuel lines or tanks that caused a slow amount of fuel leaking. It is not impossible that the fuel was being burned at a greater rate than calculated or that there were miscalculations; but I have run the numbers extensively and; even assuming the highest burn rate in the POH; should have left me with a few gallons each for the main tanks. It is impossible to know right now until the aircraft is inspected.