These two parts can increase the gear ratios to compensate for bigger tires, or to increase launch velocity, while compromizing fule economy. The increase (like 5.38:1 = DriveShaft spins 5.58 times per one tire rotation), will cause higher RPMs at lower speeds, making your maz safe speed in the 60Mph range (Most 3sp auto Transmissions).
The 3sp auto ratio is lower, while the Manual transmissions are already in the 5.xx:1 range.
By replacing a 3SP auto trasmissions with a 4sp auto, and then changing
the ring&pinion gears keeps your RPMs lower in the 4th gear, rather
than high in 3rd. This combination will give most cars a run for
their money, enabeling you to launch quicker, and accelerate faster than
with standard Auto equipped differential gearing. The easiest way
to obtain the gearing would be to pull them from a manual equipped version
in a junk-yard! Cheaper than $400.00+ for the set!