File:FAA-8083-13 Fig 4-10.PNG

English: Some variometers are equipped with a rotatable rim speed scale called a MacCready ring. This scale indicates the optimum airspeed to fly when traveling between thermals for maximum cross-country performance. During the glide between thermals, the index arrow is set at the rate of climb expected in the next thermal. On the speed ring, the variometer needle points to the optimum speed to fly between thermals. If expected rate of climb is slow, optimum inter-thermal cruise airspeed will be relatively slow. When expected lift is strong, however, optimum inter-thermal cruise airspeed will be much faster. [Figure 4-10]