I went to a ford dealer near me asking about 3.73 gears and they said they can't recalibrate the speedo. That you have to get a tuner along with the gears. They said it would be off by about 40 miles per hour if you didn't. Not sure if he didn't know what he was talking about or if he's right.
My dealer said they couldn't recalibrate for 3.73 because they don't offer it as an option for the 3.7. They could re tune for the 3.31 because Ford offers it. Unfortunately anything other than 2.73 and 3.31 you have to install a tune.