Changing Your Speedo GearKeeping tabs on your speed and mileage
Improving the overall performance of your favorite ride by changing rear-end gears is common, but it's not the only gear change required. Inside the tail housing of your manual or automatic transmission is another gear, which controls your speedometer and odometer. When you change the rear-end ratio, it throws off the gear in the transmission, so the speedo will read faster or slower until you get the correct trans gear. The various transmission gears are usually plastic and are color-coded to the ratio they correspond to. There is a great physical difference, as show here, comparing the early Super Turbine 300 Powerglide gear with a Muncie four-speed gear. Both are for 3.55:1 rear-end ratios; the four speed's is maroon and the Powerglide's is tobacco brown in color, but the shape change is dramatic.
Most manufacturers still stock (or can special-order) whatever transmission gear you need, but it might take a few days to get it. If you're planning on changing rear gears, you might order the correct trans gear and do the change simultaneously. The installation is normally simple and no special tools are required. The gear resides inside the tail shaft where the speedo cable screws to the trans. Normally there is a plate holding the gear in place, and the square end of the cable slides into the end of the gear. Here's the basic change-out; yours might be slightly different, but the principal is identical.
After unscrewing the speedo cable from the gear housing, remove the small bolt(s) holding the retaining plate in position. The housing containing the gear has an O-ring (shown here) to prevent transmission fluid from leaking out. It might take a little jiggling to pull the housing free, as the seal is tight. Be prepared for a little trans fluid to leak out when the plate is removed. Having a rag or drain pan under the tail housing will prevent a mess. This Super Turbine 300 only leaked about half a cup of fluid. Some transmissions might spit up a little more, so be prepared and work quickly.
The original gear is removed after the housing is taken off. It simply is pulled out and the new gear inserted. Trans fluid lubricates the gear, so no additional lubrication is required. Observe the way the original gear is fitted into the trans housing. It meshes with another gear in trans housing, which spins it and the speedo cable, registering your speed at the dash. Pretty simple in a high-tech world. The procedure is the same for either manual or automatic transmissions that have an external speedometer cable. Someday these gears will no longer be available, so if you're planning on keeping your car for a long time, get an extra gear just to be safe. Also keep the original gear in case you ever switch back to your original rear-end ratio.
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