The subject of "why we don't have car engines in a plane as they would obviously be so much more reliable" has been done to death on every pilot forum going
As Fuji suggests, one cannot do a direct comparison for a number of reasons.
But there are also others.
Car engines are designed to deliver their rated HP at high RPM, 5000-7000, while direct drive aero engines deliver a similar power at much lower RPM, say 2500 (my car and my plane are both 250HP, funnily enough) so the aero engine has to deliver an appropriately much higher torque (HP=rpm*torque). On an engine, everything incl crank stroke being equal, torque comes from piston surface area which is why my IO-540 is 8.8 litres whereas my car engine is only 3 litres.
But there is no way one could make the IO-540 rev at 7000rpm. It would shake itself to bits, I reckon. They have enough problems with dynamic stress on the crank at 2500rpm and have to add movable balance weights to reduce this.
So, the engine designs are very different.
I reckon every IO-540 would make TBO without any work, if it ran constantly at say 65% power, with good airflow. There are plenty that do make TBO in normal operation.
But I just don't believe that a car engine would run for 2000hrs at 65% power without something breaking. Not mechanically in the engine but on the ancillaries e.g. cooling hoses.
The basic mechanical reliability of some car engines is awesome. I gather Toyota spent USD 400M developing a certain 4 litre V8 engine for the U.S. market (as used in e.g. the V8 Lexus Soarer) and one just doesn't see any investment like that in aviation. But one cannot escape the fact that 99.9% of these engines will still spend 99% of their lives doing 10-30% of rated HP and that is bound to massively slant the figures.