Whenever you change the rear end gears to another ratio (e.g. 2.73's to 4.10's) you have to change the speedometer gear too. Why? Because when you change the rear end gears you also change the ratio between how fast the speedo gear is spinning and how fast the car is actually going.
Here is a chart to figure out which speedometer gear you will need.
*With these gears a 23 tooth speedo gear can be used, but isn't accurate. The only way to get these ratios completely accurate is by either changing the transmission drive gear, or by using a speed reducer gear box which is basically an adapter that lets you use different gears.
**To use 4.10's (or a higher gear) with an AOD you'll need an adapter,and to use 4.10s or higher with an AODE you'll need an aftermarket computer chip.