tundra uses adaptive memory. means it adjusts to the way you drive. it will learn if you drive like a grandma, and use less fuel, and if you drive like a racer, it will deploy more fuel.
resetting the ECU means it will change back to the factory settings.
if you drive like a racer, then disconnect it, and continue to drive like a racer, it will not be running like it did when it remembered, untill later when all aplicable parameters and tests run, before it will relearn.
but whats the use of driving like a racer, if you are trying to save gas.
leave the battery connected, the myth is cheese. just drive it easy. and keep your tires inflaed to factory specs