The rear differential gear ratio in a car or truck can have a significant effect on performance. Making a wide change (from 2.41- to 3.73-to-1, for example) is much more noticeable than a narrower change, but the final drive ratio choice can be important.
GM developed the TH350 transmission in 1969 to be used as a standard-performance three-speed automatic transmission. With modifications, it can withstand increased power output and some heavy-duty service. The internal gearing is -- first gear: 2.52-to-1, second gear: 1.52-to-1, and third gear: 1-to-1. The lower two gears (higher numerically) provide more leverage for acceleration.
A lower gear ratio (higher numerically) also provides more leverage. With identical tire diameters, a 4.10-to-1 ratio will turn the engine and transmission faster than a 3.73-to-1 will. This applies more force to the wheels. This can be an advantage for standing-start acceleration or for towing applications.
While the "starting line ratio" of a 4.10-to-1 gear provides more leverage than a 3.73, in most street-performance applications the difference will be negligible. More leverage can make traction difficult. Also, the 60-mph engine rpm will be about 10 percent greater.
In towing applications, one ratio may place the engine rpm at an ideal location near its torque peak. Since the 4.10 ratio will move the engine speed higher by about 10 percent, this may improve pulling power, but it will also impact fuel economy.
