I'll assume you've wired the Maxim part correctly. Two questions, at this point, do you really need the mega8? You can certainly use a much less sophisticated part. Second, I would put the tachometer pickup into the input capture rather than the external interrupt pin. This way, as soon as the interrupt occurs, the counter value is latched into a register. In this case, you get the time of the event rather than the time that your ISR serviced the interrupt. There are other configurations as well such as using your input to trigger the clock signal of one of the timers, using the internal prescalers, etc. One thing you should consider is how you will handle very low speed operation such that you dont get interrupts very often.