I was actually just reading more about this. From what I understand changing the front will make big adjustments and changing the rear will make small adjustments. Changing either one only effects the final drive which is basically how much torque or speed is delivered to the wheel so you don't need to have other mods to see any affects. Use the calculator to give you a rough idea on what will change. I'm still learning about this too so I don't know how to fix the speedo issue.