The flow conservation law of belt drive is introduced and the conclusion that the tension difference between the tighten belt and the looser leads to speed difference is proved.The formulas of the slippage rate and the efficiency under the promise of the bearing frictions

air resistance

bending resistance are ignored is deduced and verified with an example.According to the conclusion

the slippage rate and the efficiency can be calculated directly without experiment while the type of V-belt and effective tension are known.The research is practical and useful.