REAR END GEARS??

+1 to that. thats how i did it when i picked up my new rear last week. 3.27 in it tho :(. Theres also an rpm to mph calculator somewhere that tells you your ratio but i cant seem 2 fid it right now.
 
Put the car in 4th, bring it up to like 4-5k and note the speed... then using your tire size and these readings we can get you REALLY close... the higher the RPM and higher the speed the more accurate the reading...