Rear End Gears

Rear end gears have more to do with the performance, speed, and efficiency of an individual automobile than does most every other piece in a car’s drivetrain. The rear end gear, more specifically the rear end gear ratio, determines how much of the engines torque is delivered to the wheels, and the potential top speed for a particular set up.

Have you ever ridden a bike with multiple gear settings? You could put it in a high gear and spin your legs a million miles per hour and hardly move the bike at all. Or you could instead put it in low gear and move 10-15 miles per hour without much movement in your legs. Of course, peddling in high gear is far easier than low gear, and using a combination of gear ratios meant that you could accelerate in a high gear, and slowly move up, keeping your legs moving at the same cadence while increasing your speed. This very primitive engineering concept is the same idea behind rear end gears and their respective gear ratios for a complex racing car, or a daily commuting vehicle.

Rear end gears are usually described by their ratio. Three to one (3:1), for example, means that for every turn of a larger gear, a smaller gear turns three times. Thus, if a drive shaft were spinning 5 times per second, it could be converted to 15 times per second with the addition of a 3:1 gear. This is the best way to maximize efficiency, since the full use of every bit of torque is met with a higher gear. However, as mentioned previously, if gear ratios are too high, too much energy will be consumed bringing the car up to speed, just as it is difficult to ride a bike from stand still to top speed in solely high gear.

Changing Rear End Gears for Performance
Cars intended for racing often have enhanced rear end gear ratios. For the drag strip, a driver would employ a high gear intended to deliver the most torque the fastest, allowing for high speeds in a short period of time, but with a significantly lower top line speed. Alternatively, cars intended for long races use a lower rear end gear ratio. While ramping up the speed takes longer, the top speed is significantly enhanced for the long term. Thus a driver gives up a few seconds early in the race for a higher maximum speed. Over time, this additional speed makes up for a relatively slow start when compared to a quarter-mile dragster.

This can be explained with very simple mathematics:
Let’s assume two gear ratios of 3.55 and 4.1. While these are both on the upper end of the spectrum, the difference between the two numbers will still provide for a very easily demonstrable example.

We’ll also have to assume the same car in the same conditions and with the same transmission. In fifth gear of the transmission at 1500 rpm, the car with a 3.55 gear ratio will be moving at roughly 50 miles per hour. The same car with a gear ratio of 4.1 and engine revving at 1500 rpm in fifth gear would result in a speed 16% higher as result of the different gear ratio, or roughly 58 miles per hour. Each car, mind you, is using the same amount of gas, but the car with the higher ratio is getting significantly more speed out of the same amount of energy.

Rear End Gears for Efficiency
Higher gear ratios are excellent for improving long term driving efficiency. However, take into consideration your typical driving patterns. Those who drive long distances on highways should lose lower ratios, since most of their commute is done at high speed and with few stops and starts. Those who drive mostly in the city or in other places with frequent stops and starts can still afford lower gears, but should opt for a ratio at least one or two positions lower than the highway driver. If the city driver goes to a gear ratio that is too high, then the car will be very slow to accelerate, thus rendering it nearly impractical for city use.

Consider Your Choices
There are very few practical reasons for changing a gear ratio, especially for daily commuting cars. Automobile manufacturers install a rear end gear that they believe should be adequate for most people and driving styles, so to change out a gear is an implicit implication that you know better about the car than do the engineers that designed it.

While that may be true, you may know more about your own driving habits, it may also be true that there is very little real gain in adjusting the ratio unless the task can be completed yourself, with your own labor. As it stands, swapping gears is difficult, time consuming, and requires a complex calculus and incredible patience to get the job done correctly. For even the most experienced “amateur mechanics” the task can be difficult or nearly impossible, and to pay a mechanic to complete the job would cost upwards of five hundred dollars for their skilled labor. At that price, one must consider carefully how much their willing to pay for efficiency, as it is likely that many other, less expensive modifications can be made to generate similar cost savings.

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