binooetomo:
2. Use Frequency-to-voltage IC and feed the output to Analog pin to read the current engine RPM
You are chucking resolution out the window doing that. The max the analog pin can read is 1023, so for 6000 revs a second you would not be able to tell the difference between 5999 and 6002. Just hook it up to the timer input. Simple and elegant. Have an overflow interrupt to handle when the timer overflows.