It must be because the potatoes are ready for harvest.
That era of Toyota motors had very consistent mpgs either on or off the highway. Was the mileage poor and now it is reasonable, say 23 for the 4 cylinder? Or do you now get 30?
Are you checking the mileage by filling it up to the top and measuring miles with the odometer and then refilling it back up to the top and use the pump reading to figure mpg? this is the only correct way.
Is the exhaust note louder? Maybe you blew out your catalytic converter. If that gave you 5 mpg I would be cutting mine off too.
I suspect something bad was happening and it fixed itself so that you now have a good running engine. Nah, its just your excessive starch intake.