If you are changing the rear gear ratio, you will notice that the speedometer will not indicate the true speed. Just in front of the transmission tailshaft housing is where the speedometer cable screws into the transmission. If you unscrew the cable, you will see a screw holding in the gear cover which needs to be removed in order to take out the driven gear. The gear slides into the housing and only requires a slight twisting motion as you pull it out. On the gear surface, the number of teeth will be molded onto the surface. See picture.
To find the proper gear for the new rear axle ratio, use the following formula:
New speedo gear=(new axle ratio/old axle ratio)(old speedo gear)
For example changing from a 2.73 to a 3.08 the formula would look like:
New speedo gear=(3.08/2.73)(37)=42
A local transmission shop can do this for you since they have a wide selection of gears. If you are changing rear gears with a large difference say from a 2.56 to a 3.73, you will need to change the tailshaft drive gear which drives the speedometer gear. You can access the tailshaft gear by removing the driveshaft and then the tailshaft housing. You will see the tailshaft gear on the shaft held in polace by a small spring clip which is easily removed. Slide the gear off the shaft and you can install a new one on from your new set, drive and driven. The tailshaft gear will accomodate a small range of speedo gears so long as the rear axle change is not drastic. All that needs to be changed is the speedo gear. If you pull a rear axle at a salvage yard, don't forget to get the tranny speedo gears or you will have to hunt/buy another set.
Here is another formula to find out driven gear given drive gear and relevant info.
Driven gear teeth=Drive gear teeth(axle ratio)(tire rev/mi)/1000
Tire rev/mi=20168/tire diameter in inches.
For example 28" tall tire will have 720.29 rev/mi.
Here are the Chevy part # for speedo gear only: