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We upgraded our T-5's overdrive ratio with Astro Performance's A-5 0.79 Fifth Gear kit, which replaces the stock gears splined onto the main and countershafts. Note the thicker gear teeth and shallower helical angle on the Astro gears compared to the stock overdrive gears, which greatly increases the strength of the gears and reduces the side thrust against the transmission case. Replacing the Fifth gearset is a simple swap that you can do without opening up the main case or using any special tools beyond a pair of snap-ring pliers. We actually did it in about an hour at the track using the bed of a pickup truck as a bench.