When the T-43 was cancelled, the Soviet command made the difficult decision to retool the factories to produce a new model of T-34 with a turret ring enlarged from 1,425 mm (56 in) to 1,600 mm (63 in), allowing a larger turret to be fitted. The T-43’s turret design was hurriedly adapted by V. Kerichev at the Krasnoye Sormovo Factory to fit the T-34. The resulting new T-34-85 tank had a much better gun and finally, a three-man turret with radio (which had previously been in the hull). Now the commander needed only to command the tank, leaving the operation of the gun to the gunner and the loader.
Although a T-34-85 was still not a match for a Panther, the improved firepower made it much more effective than before. The decision to improve the existing design instead of tooling up for a new one allowed the Soviets to manufacture tanks in such numbers that the difference in capabilities could be considered insignificant. In May 1944, the Wehrmacht had only 304 Panthers operating on the Eastern Front, while the Soviets had increased T-34-85 production to 1,200 tanks per month.
Source: T-34 85 on Wikipedia