Jack the back end of your mustang up and rotate the hub and note how many revolutions the driveshaft does in relation to one revolution of the hub. BTW rear axle code HZ is a 3.08:1 limited slip which is kinda weird because I thought that our cars came with a base or 2.73 or an upgrade of 3.27 (what I have)
. Anyhow if someone upgraded the rear from 3.08 to 4.10 and didn't correct the speedo gear then you could have a significant variation but I'm not sure about that much of a disparity. Ken