depends a lot on maintenance...most of the loss is due to loss of compression...
as the drivetrain and bearings 'loosen up', they actually consume less power...
here's a Cummings engine...
12k mile oil changes, 300k miles, almost no power loss
in fact at power peak of 2100 it GAINED power!
with 25k miles oil changes, and only 250k miles, it loses a lot of power ~20%
