Just a note. A speedo is never off by a fixed amount. Remember, at 0 mph, it's right on. At 100 mph, it's way, way off. You have to calculate the correction by the ratio of the gears in the speedo compared to the differential gears and the tire diameter.
A good trans shop can give you the right skinny. I had the same problem in my Vette when I changed from 2.whatever to 3.55. The trans shop sent me a drive and a driven gear. I plugged them in, and everything was fine.
