If you want to do this, you should put a gear box between the engine and generator. Sure your car engine can run at 3600 rpm (I've seen exceptions, but they are rare), but typically it is most fuel efficient when run at closer to 2000 rpm at high load. At lower loads lower RPM is more efficient (but not as efficient as high load at 2000 rpm) - but your car probably doesn't have a large enough cooling system to handle high loads for very long, so perhaps run the engine at 1800 rpm and use a 1:2 gear box to get the right rpm for the generator, which makes design simple and is "close enough" to ideal without needing to see the exact efficient/power/rpm curves of your particular engine (a 3d graph).