I was wondering if my car with 40k miles is less powerful than when it was new so the question is… how many miles does it take for a stock engine to start losing its peak power and what are the first things to change to restore its full potential?

 

Possibly, things like worn spark-plugs and clogged air filters can result in small power loss over time. Otherwise a well maintained engine will provide near factory power levels indefinitely until the rings wear to a point where combustion gasses can leak by excessively, I don't see this occurring until over 200K miles though.

 

you may even have more power since everything has loosened up a bit.

 

True, if you maintain your car, your vehicle should be at optimal power all the time, until engine failure starts to show up! Keep the air filter clean, change your oil and coolant as necessary.... you should be maintaining power. Clean those fuel injectors too... don't know how that works with direct injection, however.