An adjustable airless tire with an air sac within a wheel includes an air sac located within a wheel to transfer a load of a vehicle from a ground contacting area of a tire to whole walls of the tire through a plurality of spoke structure of rods that connect with the walls of the air sac and the tire. A sensor detects a tension of the tire and a variation of a tension during driving. A control component adjusts the tension of the tire according the load of the vehicle and road conditions to improve its fuel efficiency, comfortability, controllability and safety. A rim of the wheel keeps an appropriate distance from the tire to limit the maximal level of upward movement of the rods. A transversely curved tire mounted to the rim of the wheel to form closed space between the tire and the wheel.