drives his diesel Citroens up to 350,000 miles before changing the car and even with those miles on the clock, the thing still sounds as good as new and he rarely has any engine problems. At an average speed of 50 mph (which is quite high average) that equates to 7000 hours
Things is a car only requires what 30-40(?)HP to run a 50 MPH which is about 20% of their maximum.
Airplane engines run at leats at 65% of their maximum.
That Citroen engine would be amazing if it would run 3500hrs at an average of 100MPH, but it won't.
With a small airplane a gearbox adds weight. weight is bad as it reduces usefull load, performance etc etc etc.
Light aircraft usually do not have an abundance of excess performance to begin with. So you are generally stuck with an engine that runs in the same RPM range as the propeller which is 2500-3000-ish.
Although there are some succesfull "gearbox" engines around:
DA40/42 w/ Thielert or Austro
Cessna 421, some models had geared engines.
Problem is aviation gearboxes do not have variable ratio.
Lets take a little example with the VTEC engine assuming we have a 2:1 gear box ratio;
VTEC 4000 RPM , propeller 2000 RPM
VTEC 8000 RPM , propeller 4000 RPM
And this is what happens when the propeller overspeeds
YouTube - Relentless mayday 9_14_2010_slow motion.mp4
Source:
EAA News - Reno 2010: Relentless Loses Prop in Flight