Gear replacement for a speedometer requires jacking up the car, disconnecting the speedometer cable, removing the speedometer gear housing, replacing the gear, and reassembling. Park the vehicle on a flat surface, and put wheel chocks around one of the rear wheels before beginning the replacement.
Jack up the front end of the vehicle high enough to place a jack stand under the front axle, then lower it onto the stand. Make sure and follow the jack manufacturer's safety instructions, because you are working under the car.
Crawl under the vehicle, and locate where the speedometer cable attaches to the transmission. Place a drain pan under the spot, and wrap a cloth around the cable fitting. Usually you can unscrew it by hand, but if it sticks, twist it gently loose with a pair of pliers.
Remove the bolts connecting the speedometer gear housing to the transmission. Wiggle, or gently pry, the housing free taking care not to damage the O-ring seal. Once the housing is released, transmission fluid drains freely through the hole, so make sure the drip pan is in the proper position to catch it.
Note the orientation of the old gear before removing it and installing the replacement. Be sure the O-ring seats properly against the transmission case when reattaching the housing, and secure the bolts tightly. Reattach the speedometer cable so that it is finger-tight. Refill the vehicle with transmission fluid.