Altitude vs Engine Power

Do you know that your car will have a reduction in horse power when climbing hill?
I believe not many people know about this fact.
the fact is that when you climb to a higher altitude, the lower the air pressure.
Engine combustion chamber needs some external air pressure to push air into the chamber.
At higher altitude, less air enters engine combustion chamber, and system inject less fuel into the chamber to maintain air-fuel ratio.
This in return gives less powerfull combustion, hence, less power.
If a car at Sea level can churn out 300 horse power, it can have its power reduced to 200 horse power at higher altitude. About 30% reduction in power.
This is interesting.
This Video illustrates the theory: