Recalibrates or repairs the speedometer drive on a GM TH350 or Powerglide, with driven gears from 17 to 22 teeth and drive gears in 7, 8 and 10.
If the speedometer in your Turbo 350 car or truck quit, bounces, or reads 8 mph fast since you put in 3.73 gears or taller tires, the fix lives in the tailhousing. This 21-piece kit gives you six driven gears (17 through 22 teeth), three output-shaft drive gears (7, 8 and 10 teeth), a steel bullet housing, O-rings and seals, the retainer, the drive gear clip and bolts, so you can repair a stripped gear and recalibrate in the same job. It also covers the Powerglide.
The TH350 is a mechanical-cable speedometer transmission. A plastic drive gear snaps onto the output shaft and is held by a small steel clip that sits in a hole in the shaft. That gear turns a driven gear on a short shaft that rides in a bullet housing in the side of the extension housing, and the speedometer cable threads onto the bullet. Because the drive gear turns with the output shaft, the reading depends on rear axle ratio and tire size as much as on the transmission itself. Change either one and the speedometer drifts by a fixed percentage at every speed. The gears in this kit are glass-fiber reinforced plastic, which holds its tooth shape better in hot fluid than plain nylon.
GM's own TH350 leak diagnosis lists a damaged speedometer gear O-ring as a case leak source, so if the gear is out anyway, the new O-ring is cheap insurance. A leak at the rear of the unit can also be the tailhousing seal or the square-cut extension seal; our TH350 fluid leak guide walks through where to look.
First rule out the cable: unscrew it at the bullet, spin the inner core by hand and watch the needle. If it moves, the cable and head are fine and the problem is in the transmission. Then pull the driven gear and look at the teeth. To size a new gear, use the standard formula: driven teeth = drive gear teeth x axle ratio x tire revolutions per mile, divided by about 1,000 (some guides use 1,001). Tire revolutions per mile are roughly 20,168 divided by the tire's loaded diameter in inches. As an example, an 8-tooth drive gear, 3.73 axle and a 28-inch tire works out to about 21.5, so a 21 or 22 tooth driven gear gets you close. More teeth on the driven gear slows the reading; more teeth on the drive gear speeds it up. Check the result against GPS or a measured mile.
This kit fits the GM TH350 (Turbo Hydra-Matic 350) and the Powerglide with a mechanical speedometer cable. It does not apply to units with an electronic speed sensor in place of the cable. The tooth range covers most common street axle and tire combinations, but very tall gearing can call for a count outside 17-22, so run the formula before you order. If you are not sure which transmission you have, send us your year and VIN, and see every part we stock on the TH350 collection page.
The driven gear comes out with the unit in the car: remove the cable, take out the retainer bolt, and pull the bullet straight out. Lube the new O-ring with ATF, fit the gear, push the bullet home and torque the retainer bolt to the factory 12 lb-ft. The drive gear is a different job. It sits under the extension housing, so the tailhousing has to come off (four bolts, 35 lb-ft on reassembly) and you need a new square-cut extension seal. To swap it, press the retaining clip down, slide the old gear off, then line the slot in the new gear up with the clip and push it on until it locks. That makes a drive-gear swap a natural add-on to a reseal or rebuild with a TH350 overhaul kit. Top off with Dexron-type ATF; the TH350 fluid guide has the spec and refill amounts. If the pan comes down too, the finned aluminum pan kit adds a drain plug for the next service.
| Pieces | |
| Driven gears | |
| Drive gears | |
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| Gear material | |
| Fits | |
| GM gear numbers referenced | |
| Calibration |
Doing this repair right usually means replacing these at the same time. Same transmission, ships together.
Yes. The TH350C uses the same cable-driven speedometer layout, a clip-on drive gear on the output shaft and a driven gear in a bullet in the extension housing. Check that your unit still has a cable outlet and not an electronic sensor.
Multiply drive gear teeth by axle ratio and by tire revolutions per mile, then divide by about 1,000. Round to the nearest gear in the 17-22 range and check against GPS. More driven teeth slows the reading, fewer speeds it up.
In most cars you can remove the extension housing in the vehicle after dropping the driveshaft and supporting the transmission, but it is more work than the driven gear. Replace the square-cut extension seal and torque the bolts to 35 lb-ft.
No. Spin the cable core by hand at the transmission end. If the needle moves, the cable and head are fine and the driven gear or drive gear is the likely problem. If it does not, the cable or the speedometer head has failed.
The TH350 was specified for Dexron ATF. Only a small amount is lost when pulling the bullet; check the level hot in Park after the road test.
