Although many works are published about the achieved advancements in the manufacturing of the catalytic converters (CC) system for vehicle engines and their testing under laboratory conditions, there is a lack in the published research about the mileages influence on their conversion efficiency (CE). Dependence of dual-brick CCs' CE in real-world driving conditions on vehicle mileage is studied for the first time. The CC tested are dismantled from the vehicles with mileage from 0 (new one) up to 150,000 km. The studied CC are evaluated at the engine test bench containing a dynamometer coupled with a spark ignition engine suitable for this type of CC system. Measurements of CC efficiency are performed at four different engine operation regimes: two loaded regimes and two non-load regimes - low and high speed idling. It is found that the oxidation of CO and HC at all four tested regimes took place almost totally in the first CC. Under higher engine loads the conversion of NO also took place mainly in the first CC. The overall efficiency of conversion reaches values above 99% for most of the tested systems. No significant degradation of CE is found in the studied vehicle mileage range. CE of the CC system that has accumulated the highest mileage, of almost 150,000 km, reaches values of 97-99% for CO, 86-99% for HC and 64-99% for NO at the loaded testing regimes. The smaller values are obtained for the low load regime when the CC temperature is decreased. Considering that the European legislation imposes a durability of 160,000 km for the CCs, the results obtained in this work show that for the mileage of 150,000 km CC keep a very high CE rate. The CC system that included an aftermarket non-genuine CC is the only one, where significant catalyst deactivation is observed. Registered values of the backpressure do not show any of the tested CC systems as being clogged.