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Gearing and horsepower are essential components of the performance of a vehicle. However, many car owners and enthusiasts wonder whether gearing affects horsepower. Luckily, this article aims to provide more insight into how gearing and horsepower relate.

Many automakers and professionals in the automotive industry try to explain if gearing affects horsepower and how the two aspects relate to each other. Horsepower, gearing ratio, top speed, and how hard the engine has to work to achieve that high speed determines a vehicle’s overall performance. Automakers have to balance a vehicle’s performance and fuel economy when determining final gearing ratios. However, to understand gearing, you have to comprehend gearing fully.

What is horsepower?

Horsepower combines the concept of time and work. Initially, from James Watt, the inventor of steam engines, horsepower denotes the amount of power an engine can deliver. A dynamometer measures horsepower by testing how much strength (torque) and power the engine has before the dynamometer breaks or stops it; thus, a vehicle’s braking action.

The term horsepower denotes a car’s power and performance; hence the phrase, the greater the horsepower, the higher its performance. Maximum horsepower determines the top speed of a vehicle. Similarly, horsepower (hp) is usually in Kilowatts.

To calculate horsepower, you need to consider the amount of weight involved, distance, and how long it takes for the vehicle to cover that distance. Therefore, horsepower is equivalent to torque (lb-in) multiplied by speed (rpm), divided by 63025.

Does gear reduction increase horsepower?

Gear reduction occurs when an engine’s output speed reduces or the torque increases. It refers to the rotational speed of the engine when it is divided by a gear ratio greater than 1:1. To arrive at a gear ratio greater than 1:1, interlock a smaller gear with a fewer number of teeth (reduced size) with a larger gear with a higher number of teeth. In most cases, gear reduction reduces speed and increases torque, while in other circumstances, it increases speed and reduces torque. As a result, there are instances when gear reduction increases and reduces horsepower.

Often, gear reduction increases speed and reduces torque in the generators of wind turbines, whereby a relatively slow turbine blade speed converts to a high-speed generator, with the capacity to generate electricity.

Typically, gear reduction reduces the speed of rotation but does not affect horsepower. Automotive machines produce power over a limited number of rotations, measured in RPMs. An engine has a characteristic ‘torque curve’ and ‘power curve,’ which usually indicates the maximum torque and power available in the particular vehicle for a specific RPM. Similarly, these curves show the perfect rpm the engine should operate for given load capacity.

How does gear ratio affect speed?

The primary function of gears is transmitting power from one part of a vehicle to another. In vehicles, gears transfer power from the crankshaft (a rotating axle that obtains energy from the engine) to the driveshaft, which ultimately sends the power to the vehicle’s wheels.

When a series of gears transmit power, it can significantly affect the vehicle’s rotational speed. Higher gear ratios (lower numerical axle ratio) provide better acceleration/torque, and lower gear ratios allow higher top speeds and better fuel economy. A vehicle with a high gear ratio runs faster to produce more power and achieve a given speed. Lower gear ratios ensure the engine runs more economically to maintain a given speed.

Gears increase speed when you connect two different gears. The first gear has to be bigger in wheel-size than the second one, meaning it has more teeth when compared to the second one. Therefore, the second gear will turn much faster to keep up with the first one, but with less force; thus increasing speed. On the other hand, if the second gear has more teeth than the first gear, the first gear (input gear) will turn faster than the output gear (the second one), reducing the overall speed of the vehicle.

How does gear ratio affect torque?

Torque refers to the amount of ‘rotating force’ that may or may not result in motion, produced by an engine’s crankshaft. It is the amount of force multiplied by the length of the lever through which it operates. The more torque produced by a motor, the higher its ability to perform work. Newton-meters and pound-feet measure torque.

When using a series of gears to transmit power from the engine to the wheels, the gear connected to the engine is the input gear or driver, while the gear attached to the wheel is the output gear or the driven. Gears found in between the input and output gears are idler gears. Gear ratio refers to the ratio of output torque to the ratio of input torque.

Gear reduction reduces speed and increases torque. High gear ratios (lower numerical rear axle ratios) provide more torque and acceleration, meaning a vehicle’s engine has to run much faster to achieve a given top speed; hence, increased fuel economy. Alternatively, low gear ratios allow easy attainment of high speeds and better fuel economy.

What is rear axle ratio?

Rear axle ratio is the overall number of revolutions the driveshaft or output shaft makes to spin the axle one complete rotation. It refers to the number of gears in a truck’s differential, the machine that links the driveshaft to the rear axle and then to the engine. Expressed in the form of ratios, such as 3.42:1, this number denotes that the driveshaft turns 3.42 times for each wheel turn. However, this ratio is usually referred to as 3.42, or simply ‘three forty-two.’

Differentials and axles have gears that allow power transfer from the driveline and engine to the wheels of the truck. The transmission and engine turn a driveshaft that’s linked to a durable pinion gear within the axle. In turn, the pinion gear links and turns the ring gear, which is connected to a differential case that has many smaller gears that send power to the wheels.

Difference between 3.55 and 3.92 axle ratio

Automakers manufacture vehicles with different axle ratios, motor transmissions, drivelines, and torque. The different gearing options make all the difference in your vehicle’s performance; thus, the need to understand axle ratios. If you do not plan on using your truck for any heavy-duty hauling or for pulling trailers regularly, standard axle ratio will work perfectly in your truck. However, if you will use your vehicle for hauling heavy loads or pulling a trailer, it is advisable to understand axle ratios and how it affects your truck’s performance and overall fuel consumption. For instance, you should be able to explain the difference between 3.55 and 3.92 axle ratios.

What is a 3.55 axle ratio?

Axle ratio refers to the gears in the truck’s differential, which is a mechanical device that connects the driveshaft to the rear axle and then to the engine. It is the number of revolutions the driveshaft or output shaft makes to the rear axle. Axle ratios are numerically expressed in a ratio, representing the number of teeth on the ring gear to the number of teeth on the pinion gear.

For example, a truck with a 3.55:1 gear ratio means that the output shaft has to turn 3.55 times for one complete rotation of the rear axle. Similarly, a vehicle with a 3.92:1 gear ratio means the driveshaft has to turn 3.92 times for one full wheel spin. Often written as only 3.55, instead of 3.55:1, both forms of axle ratios mean the same thing.

How do I determine my axle ratio?

Optional gear ratios affect the truck’s hauling capabilities, fuel economy, and overall drivability. One way to calculate gear ratio is to put your vehicle in neutral and raise the rear wheels off the ground by sliding a floor jack under the rear of the vehicle. Locate where the driveshaft links to the front of the rear axle and rotate the tire to make one complete revolution. Count the number of revolution the output shaft makes in one tire revolution. If you find it makes three and a quarter rotations, then your gear ratio is approximately 3.25:1.

Another gear ratio calculation formula is to count the number of teeth on the ring gear and divide them by the number of teeth on your pinion gear. However, you will have to take apart your truck’s differential to access the ring and pinion gear.

If you do not want to disassemble your differential, you can find your axle ratio in the owner’s manual or repair manual. You can also locate your vehicle’s axle ratio by reading it off a sticker near the door latch on the front door on the driver’s side. Sometimes, it’s written as a code, but such codes should be in your owner’s manual.

What is the best axle ratio?

Automakers pick standard axle ratios that balance out the vehicle’s performance, fuel economy, and capability. By choosing a final drive axle ratio which lowers the engine’s RPM in normal operating conditions, manufacturers meet standard government mileage guidelines with ease. On the other hand, differing axle ratios gives cars more towing ability by increasing the amount of available rotational power as delivered to the axles from the engine.

If you plan to use your truck for towing or for carrying heavy loads, you should get a truck with the highest possible axle ratio. As a result, you will reduce fuel economy by about one mile per gallon. Another factor to consider when choosing a good axle ratio is the drive cycle.

If you intend to drive at high speeds on the highway, you may want to purchase a vehicle with a lower gear ratio to improve fuel economy and reduce the engine’s RPM. If you will be driving your truck in rough terrain and at low speeds, a vehicle with a high gear ratio will provide a better driving experience without much damage to the final mpg number.

Axle ratios range from 2.26 to 6.50, whereby the higher range is best for vocational lower speed segments. However, for highway driving, axle ratios between 2.26 and 3.42 are ideal for better fuel economy.

Importance of knowing your axle ratio

One of the significant benefits of understanding the axle ratio is that it gives you a mechanical advantage. There will be a trade-off in rotational power, that is, between torque and speed. When the rear end gear ratio is 1:1, the amount of speed and torque is similar. Therefore, when you increase the axle ratio, you reduce the amount of torque and significantly increase speed, and if you reduce the axle ratio, you reduce speed and boost the torque.

Calculating a vehicle’s axle ratio involves counting the number of teeth on the pinion gear as well as the ring gear and dividing the ring gear’s teeth number by the number of pinion gear teeth. Luckily, modern trucks come with a sticker on the outside of its differentials, where you can easily find axle information.

Alternatively, you may also opt to lift the vehicle in the air and put it on neutral to ‘roughly’ see how many revolutions the driveshaft makes for a complete axle rotation. This observation will provide an estimated figure since you will find the closest axle ratio to your number. For example, if you count four revolutions, then you most likely have a 4.10 axle ratio.

Is it better to have a higher or lower gear ratio?

Axle ratio directly affects a truck’s towing capacity, fuel economy, and torque. A truck with a high axle ratio number means more towing capacity, decreased fuel economy, and a lower top speed. More mechanical power of the engine’s available torque is sent to the rear tires, improving off-the-line acceleration and more power for pulling heavy trailers and hauling hefty loads.

A truck with a low axle ratio number means the vehicle will go faster and improved fuel economy, but lower towing capacity. The vehicle’s engine will work less when speeding on highways. A vehicle with a lower numerical axle ratio is said to have a high gear or high rear end ratios while vehicles with a higher numerical ratio have low gear or low rear end ratios. Therefore, trucks with high gear ratio are best for cruising while those with low gear ratios have fast acceleration.

Whether it is better to have a higher or lower gear ratio depends on what purpose your truck will serve. If you are looking for a truck to carry heavy loads or pull heavy trailers, you will need one with a lower gear ratio as it has more towing capacity and vice versa. A small engine truck has a low gear ratio to give it adequate acceleration, while a high-powered engine truck has a high gear ratio to provide relaxed cruising at higher top speed.

Similarly, trucks with short tires require a high gearing ratio to travel the same distance as a taller-tired truck, meaning their engines need to produce more power and this is only possible by increasing the gear ratio.

Would raising a car gear ratio (from 2.73 to 3.73) affect fuel?

Gear ratios directly affect fuel consumption. At a given speed, high gear ratio positively affects your vehicle’s fuel economy and vice versa. By raising your car’s gear ratio from 2.73 to 3.73, typically you reduce your engine’s fuel consumption since you increase your car’s normal operating rpm during driving, translating to lower fuel economy.

Higher gear ratios (lower numerical values) mean the engine runs faster to achieve a given top speed; thus, better acceleration/torque and high fuel economy. On the other hand, lower gear ratios allow your vehicle’s engine to run more economically to maintain a given speed; hence, better fuel economy and higher top speeds.

Changing gear ratios can lead to a drastic shift in fuel savings or increased acceleration advantages. However, installing a new set of rear end gears is one of the most challenging automotive tasks to accomplish. It requires precise measurements when lining up. If the mesh does not line up correctly, it may lead to excess friction in the driveline, whining sounds from the rear end while driving, and premature wearing of the gears.

Gear ratio fuel economy calculator

Using a gear ratio calculator helps in finding an effective gear ratio for your vehicle with new tire size. It aids in determining the best ring and pinion gear ratio for your car; thus, optimizing fuel economy, performance, and everyday driving.

Auto repair and tire shops provide information charts on gear ratios to tire size. Some other tables display the new gearing you may need to go back to your initial gear ratio when switching to either a bigger or smaller tire.

A gear ratio and a tire diameter chart give an approximation of ideal RPMs at 65mph (highway speed) for better fuel economy, increased towing power, and overall drivability. It represents a 1:1 gear ratio on a manual transmission, ensuring your car is as efficient as possible. Auto transmission RPMs are higher than those in manual transmissions and overdrive transmissions tend to be a bit lower.

Another way to easily calculate gear ratio is multiplying .12 by your new tire size, for example, .12*38”=4.56. You may also calculate rear axle gear ratio by dividing pinion gear tooth count by ring gear tooth count.

Tire specifications have a three number format (275/75R16). In this case, 275 is the tire tread width in millimeters, 75 refers to ASPECT (percentage of tread width to yield the sidewall height), and 16 is the diameter of the tire. Truck and off-road tires come with specifications like 30*9.5R15, with 30 as the tire diameter, 9.50 as the tread width and 15 as the wheel diameter. In such scenarios, you do not need to calculate the tire diameter.

Furthermore, you need to determine your tire diameter when using an RPM calculator to find the gear ratio. The formula to calculate your vehicle’s tire diameter is (cross-sectional width*0.aspect ratio*2/25.4) + wheel diameter.

Conclusion

Horsepower aids in calculating a vehicle’s potential in terms of engine and other engine components. Vehicles with high horsepower are often seen as prestigious, stylish, and more expensive, as compared to those with lower-horsepower. Automakers create competition in the automobile industry by manufacturing improved quality vehicles in terms of horsepower. However, it is also vital to consider compact and mid-sized vehicles with lesser horsepower, as they work better without it. Similarly, by understanding how gearing affects horsepower, you can make informed decisions, if you choose to alter your gears.

FAQ

Which gear ratio is the fastest?

The general rule is that the lower the number of gears the faster a car will travel with the same number of engine revolutions. The problem arises in how we define these gears. You will often have the high numerical gears been referred to as low gears while the lower and vice versa. The higher gears apply when you are cruising while the lower gears are great for acceleration.

What are the benefits of geared transmissions?

When put together with other transmissions the geared ones offer higher performance when transmitting motions and forces. This gives them a higher service life which results to more reliability. Due to the smallness of gears you can install them in virtually anything unlike the pulleys and chains.

What is the best gear ratio for towing?

Typically, gear ratios intend to balance performance, fuel economy, and the vehicle’s capability. Automakers select a final drive ratio that lowers the engine’s rpm; thus, efficiently meeting the government’s mileage standards. Optional axle ratios give more towing ability by increasing the engine’s available torque delivered to the axles.

If you are looking for a truck for towing, the highest possible axle gear ratio is what you need. A numerically higher axle ratio knocks off about a mile per gallon from the truck’s fuel economy. Therefore, a car with optional 4.30 gears

What is the best gear ratio for towing?

Typically, gear ratios intend to balance performance, fuel economy, and the vehicle’s capability. Automakers select a final drive ratio that lowers the engine’s rpm; thus, efficiently meeting the government’s mileage standards. Optional axle ratios give more towing ability by increasing the engine’s available torque delivered to the axles. If you are looking for a truck for towing, the highest possible axle gear ratio is what you need. A numerically higher axle ratio knocks off about a mile per gallon from the truck’s fuel economy. Therefore, a car with optional 4.30 gears