In one of the first Dragonball Z episodes, Raditz visits Earth, he scans the power level of a normal man which was 5 Power Level, lets call it PL. (Raditz had a PL of 1200 then). After some research Superman speaks his lift at around 100 tons = 100,000 kg = 220, 462.262 pounds. An average human (non powerlifter) can lift around 40% of his body weight as max (straight up in the air while standing). An average person weighs 80 kg, 40% of 80 kg is 32. 100, 000 kg/32 = 3125, superman can lift 3125 times more than the average person = Strength of 3125 men. Remember 1 normal person had a PL of 5. 5 x 3125 = 15, 625 Superman's power level is 15, 625. In conclusion, Goku could beat Superman after training with King Kai. After that Goku could reach a 16, 000 PL with the Kaioken x 2. In the episode "Over 9000" Goku could reach power level of 40, 000 if he used the Kaioken x 4. But only with Kaioken x 2 he could reach 16, 000 and beat Superman.