Jack the car up and count the number of times the driveshaft turns per one rotation of the wheel. 3 1/2 turns = 3.55, almost 4 = 3.91's a tad more than 4 = 4.10's.... 4k at 70 seems high to me however I usually run 275/60/15's and they are about 1" taller 27" vs 27.99"

whoops forgot, you need the right speedo gear. find out what rear and check e-bay for the correct gear.