So you tossed a set of 4.10s in as your rear axle gears 'cause you want to go fast...but do you really have any idea what you're doing? According to our reader survey, plenty of you are willing to admit that you really don't.
That's why this story will cover gearing basics for first-timers as well as share some of the physics of gearing that you hard-core types may not have considered. We'll also draw on some of our own experiences to help you decide how to gear your car for the street or strip. It may help if you read the "Gearing Lingo" sidebar first.
Axle Gears: What Is a Gear Ratio?
When you hear people refer to numbers like 3.08, 3.73, or 4.10, they're talking about the ratio of the ring-and-pinion gears in the rear axle-hence, the numbers are more accurately 3.08:1, 3.73:1, or 4.10:1. The ratio is the number of teeth on the driven gear (ring) divided by the number of teeth on the drive gear (pinion). So, if the ring gear has 37 teeth and the pinion has 9 teeth, the ratio is 4.11:1. That also means that for every one turn of the ring gear, the pinion will turn 4.11 times.
What Do Axle Gears Do?
In addition to changing the direction of power flow by 90 degrees (from the driveshaft to the axles), the purpose of the rearend gears is to multiply the torque delivered by the engine and transmission. Gears can be thought of as complex levers. In other words, they provide a mechanical advantage that multiplies work-in this case, torque-to help the engine's power move the vehicle. Lower gears are like a longer lever: They provide more mechanical advantage. Higher gears are like a shorter lever: They provide less mechanical advantage. It's similar to when you use a long breaker bar instead of a short ratchet handle to remove tight lug nuts. Just like a long bar puts more torque on a lug nut, lower axle gears provide more torque to the wheels.
It's very easy to calculate the torque multiplication provided by your axle gears-just multiply by the gear ratio. For example, let's assume that the engine and transmission are delivering 100 lb-ft of torque to the pinion gear. If the gear ratio of the ring-and-pinion is 4.10:1, then the output torque is 410 lb-ft (100x4.10). Similarly, if the gear ratio is 3.08:1, then the output torque will be 308 lb-ft. It's easy to see that the lower 4.10:1 gears put more power to the ground than the higher 3.08:1 gears. Keep in mind that the engine's power has not changed but that the available torque to the tires has.
Axle Gears vs. Engine RPM
Considering that lower gears provide greater torque multiplication, it would seem that they are always the best choice for performance use. However, lower gears require more input speed (engine rpm) to produce the same output speed (tire rpm). Higher gears multiply torque less, but they require less input speed to deliver the same output speed; that's why axle ratios also determine engine cruise rpm.
Again, think of a long breaker bar versus a short ratchet. With your hand at the far end of a breaker bar (which is a longer lever, like lower gears) the job is much easier, but to turn a lug nut one complete revolution requires your hand to travel a much greater distance than it would with a smaller wrench (shorter lever, like higher gears) which has a smaller turning circle. Similarly, lower axle gears (longer lever) require the engine to move a greater distance (turn more times) per tire revolution than higher gears. To look at it a different way, if the trans is in a gear with a 1:1 ratio (like Fourth gear on most four-speeds) and the rear gears are 3.08s, then the engine must turn 3.08 times for every one rotation of the tires. Lower 4.10:1 gears would make the engine turn 4.10 times for each turn of the tire, so lower gears cause higher engine rpm at any road speed.