the closest you could get it with a gear on the speedo is a 21 tooth with the 3.90's your optimum speedo gear ratio would be .327, and the 21 tooth would put you at .333, that's only 5 thousands off. i think thats like less than 1% off
you can buy it at hanlon motorsports, steeda, you can get them from 50resto, a bunch of different places. if you want to check the calibration yourself go to www.hanlonmotorsports.com, and enter your tire size gear ratio etc. into their speedo cal.