It's not a silly question, though it has been asked before
In brief, there have been quite a few attempts to certify car-derived engines, and homebuilts have used car-derived engines with modest success. Mostly they turn out to be less reliable than the traditional aircraft engines, and may be heavier. If you search around you'll find explanations of why various engines haven't worked.
There are quite a few Volkswagen engines in homebuilt aircraft, though I think this is done primarily for reasons of cost rather than economy or reliability.
Incidentally, even though your car can work at 3000 RPM reliably, this is not directly related to power output in the same way as it is with a fixed-pitch propeller - it won't be working nearly as hard as an aircraft engine at this RPM on climbout. You would also need a gearbox to make full use of its power output (5000 rpm would destroy a standard propeller).