Standard Transmissions

Gear Ratio

It is important to note what the gear ratio is for the gears in each one of the transmissions. But before we do, let’s talk about what gear ratios actually mean. For instance, the Muncie SM465 has an effective first gear ratio of 6.55:1. Let’s break this down.

If the engine is idling at 800 rotations per minute then we can safely assume the flywheel, clutch driven disc and pressure plate are also spinning at 800 rpms. Because the input shaft of the transmission is coupled to the clutch driven disc through a set of splines, then the input shaft must also be turning 800 rpms at idle when the clutch is engaged. If we put this transmission in first gear then the input shaft will turn 6.55 times for every one time the output shaft turns. So simple math tells us if the input shaft is turning at 800 rpms the output shaft will be only turning at about 122 rpms because we divided 800 by 6.55. So the output shaft turns slower than the input shaft dependent upon the level of the gear reduction.

The other thing that happens with gear reductions is that we slow down the speed of the output shaft, but we also compound the torque. If we slowed down the output shaft by 6.55 times then we also compounded the input torque by 6.55 times. If the particular engine mated to the SM465 transmission had 300 ft/lbs. of torque at idle into the input shaft of the transmission, then the output shaft would have about 1900 ft/lbs. or more because of the gear reduction. Similar inverse concepts can be held true for overdrive gears.

The G56 transmission has an overdrive ratio of .79 or a 79 percent overdrive. This means that for every time the input shaft turns one revolution, the output shaft turns 1.26 times. Overdrive gears in transmissions help to keep engine rpms low when traveling at highway speeds. It is important to remember although overdrive gears reduce engine rpms, they also reduce engine torque instead of multiplying torque like a gear reduction does.