I see I'm late to the party, yet another thread where input capture was discussed and evidently tried:
http://arduino.cc/forum/index.php/topic,96971.0.html
How accurate is "accurately"? When you say 5.15µs, are you implying 0.01µs accuracy? It'd be pushing it to expect even 0.1µs accuracy with Timer1 running at 16MHz.
There's also a bigger picture here, namely, once a measurement is taken, what is done with it? How often do measurements need to be taken? If the intent is to measure each cycle, then that leaves precious little time to do the associated processing.