The Turbo 400 (TH400) is one of the most popular automatic transmissions in America, particularly among hot-rodders and off-roaders who value its strength and durability. Since many enthusiasts use larger than stock tires, the stock gear ratios in the transmission speedometer cable housing are unlikely to accurately reflect the vehicle's true speed. Unless you're using extremely large tires or an oddball rear end gear ratio, simply replacing the driven gear ratio should be enough to bring your speedometer back into line.
Position a set of wheel chocks in front of the front wheels of the vehicle then insert a floor jack under the rear axle. Raise the rear of the vehicle and insert a set of jack stands under the rear axle. Raising the vehicle may not be necessary to access the transmission, but it will shift all of the fluid to the front of the pan and keep it from flooding out of the speedometer gear hole.
Locate the speedometer cable and follow it to the transmission housing. The speedometer cable screws into the speedometer gear housing; grasp the speedometer cable base with a pair of channel-lock pliers and turn the cable collar counterclockwise to unscrew it.
Remove the bolt or bolts that secure the gear sleeve to the transmission tailshaft housing. Mark the gear sleeve and tailshaft housing with a permanent marker for later reinstallation. Grasp the gear sleeve and wiggle it out of the tailshaft housing.
Pull the plastic gear (the driven gear) off of the shaft protruding from the gear sleeve or pull the entire shaft out of the gear sleeve. Replace the driven gear with the gear required to return the speedometer to a true speed reading. Aftermarket TH400 gears are colour-coded; consult your manufacturer's gear ratio chart to determine the colour gear you need.
Dip the new gear into transmission fluid. Clean the gear sleeve O-ring off with a rag, moisten it with fluid and reinstall it into the housing. Reinstall the bolt and tighten it to 25ft.-lbs. Screw the speedometer cable back onto the sleeve.
Replacing the driven speedometer cable gear will allow you to correct the speedometer about 10 MPH in either direction. The standard practice for larger corrections is to remove the transmission cross-member and tailshaft, remove the plastic clip that holds the drive gear in place and replace it with a larger or smaller gear. However, the simpler solution is to purchase an external converter drive gear set with a two-piece speedometer cable. The converter gear set sits in line between the transmission and the main speedometer cable and installs in minutes without pulling half the transmission apart. Contact a converter gear manufacturer, tell them your speedometer reading versus true ground speed and they'll send you a converter gear set pre-calibrated to correct the speedometer. These gear reduction units are typically marketed as "tire correction" or "speedometer correction" boxes.