This. If you buy a brand new car, e.g. from the NCD, It'll be in its initial break-in point. When the car reaches 200km, the break-in point stops and can go for hundreds of kilometers (don't know exactly). However, somewhere around 500-1000km (again, don't know exactly or if this is correct), the car will start to loose HP slowly, which means its engine is worn off. Also, the oil looses its strength after 200km, which also equals in a small HP loss. And I'm not sure If this exists, but installing permanent aftermarket parts reduces the longevity & endurance of the engine.
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And I'm not sure why your cars have more HP while they have more mileage. Did you install any aftermarket parts? That will probably retain your HP.
