Your Classic Car Parts Headquarters
Applicable years: all
Cost: $250-300
Tinware: ring and set, differential fluid
Tip: It's best to seek out the help of an experienced technician when changing the rearend gears.
Performance gains: quicker off-the-line acceleration
Most of us are familiar with the notion that rear-axle gears largely affect your car's performance. The more gear you have, the quicker you go, right? Well, it's not always such a black-and-white case. In fact, there are tons of variables that ultimately determine the right choice for your machine.
The first thing to grasp and really understand is the actual gear ratio and what it represents. The rearend gear ratio refers to the relationship between the driven gear (ring) and the drive gear (pinion), and is easily calculated by dividing the ring gear tooth count by the drive gear tooth count. For example, if the ring gear has 41 teeth and the drive gear has 10 teeth, the correct gear ratio would be 4.10:1. This also means that for every one revolution of the ring gear, the drive gear will turn 4.10 times. The driveshaft and the rear axles have the same comparable relationship.
The easiest way to understand the operation of the rear gears is to think of them as mechanical levers. They provide mechanical leverage that multiplies torque to help the engine move the car. As the gear ratio becomes numerically higher, the gears offer more leverage to assist in acceleration. These types of ratios are commonly referred to as lower or deeper gears. If the ratio drops numerically (from 4.56:1 to 3.55:1), the gear set becomes taller and offers less mechanical leverage. The relationship between the two types of gears is like changing a tire with a socket wrench versus a long breaker bar. It requires much more force to move the small wrench (tall gears) than it does the longer leverage bar (deep gears).
Rearend gears work in the same fashion. If the engine and transmission deliver 100 lb-ft. of torque to the gear, and the gear ratio is 4.56:1, the output torque to the rear wheels would be 456 lb-ft. (100 x 4.56). With a taller gear ratio of 3.08:1 in the same car with identical torque, the output torque would drop considerably to 308 lb-ft (100 x 3.08). This is why deep gears are always so effective on the dragstrip; it's free torque.
However, there is a compromise that comes with running a lower gear set. At the top of that list is engine cruise RPM. Lower gears require more input speed or revolutions of the motor in order to achieve the same output speed at the tire. A taller gear will not make as much torque, but it requires less engine RPM to make the same tire RPM.
Using the aforementioned ratios, the engine would turn 4.56 times for every one revolution of the tire with the lower ratio, 3.08 times with a taller ratio. This has a big affect on your highway cruising speeds. By swapping out a 3.08:1 gear with a lower 4.56:1 gear with a 26-inch tall tire, your cruise RPM at 65 mph would jump from 2,600 to 3,800!
When it comes to gearing, there are plenty of options. Most dragstrip-bound cars usually run a gear in the 4.10:1 neighborhood. If the car is heavy, or the peak of the engine is at the upper end of the RPM scale, lower gears (numerically higher) may be necessary. If the car is a little on the light side, yet still makes good power, you can get away with running a taller rear gear ratio.
Naturally, there is less torque required to move a lighter object than a heavy one. For top speed, tall or shallow gears are used to reduce the engine RPM versus the actual road speed. These types of cars may not have quick, low-speed acceleration, but selecting the right transmission can easily compensate for the lack of bottom-end grunt.
There are many other factors that affect the gear ratio and vice versa, including transmission type and individual transmission gear ratios, tire diameter, and torque converter stall speeds (automatic). These should all be heavily considered before changing the existing rear gear ratio.