How to choose the correct gear ratio

Written by david somerset
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How to choose the correct gear ratio
Choose the correct gear ratio for the type of driving you will be doing. (car stick shift image by Dimitar Atanasov from Fotolia.com)

The gear ratios in your car's transmission and axle are used to multiply the power produced by the engine. Depending on what you intend to use your car for, having the correct gear ratio is essential. A gear ratio that provides maximum power for drag racing will not work well for a car that you intend to drive down the highway. It is important to choose the correct gear ratio for the type of driving you will be doing.

Skill level:
Moderate

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Instructions

  1. 1

    Determine a proper rear end gear ratio. This ratio is determined by the number of teeth on the ring gear and the number of teeth on the pinion gear. To get the ratio, divide the tooth count of the ring gear by the tooth count of the pinion gear. 4.10 to 1 is a common rear end gear ratio in drag racing cars. One way to achieve this gear ratio is to use a ring gear with 41 teeth and a pinion gear with 10 teeth. Dividing these gives us a ratio of 4.10 to 1.

  2. 2

    Decide what gearing you want to use in your transmission. This ratio is determined by the number of rotations of the engine versus the number of rotations of the driveshaft. If first gear in your car has a 3 to 1 ratio, this means that the driveshaft will spin once for every three rotations of the motor. Higher ratios will give you a higher top speed, while lower ratios will give you more power and make your shift points closer together.

  3. 3

    Consider the size of the tires on your car. The formula which relates tire size to gear ratio is, "Gear ratio = (RPM x tire diameter) / (MPH x 336)." While your tire size will not affect your gear ratio directly, the relationship between the tire size and the gear ratio will affect the RPM at which your engine runs and the speeds at which your car will travel.

  4. 4

    Find a balance between transmission gearing and rear end gearing that will give you the performance you want in combination with the size of the tires on your car. The formula from Step 3 can be manipulated to give you an idea of how your car will perform with regard to RPM or MPH. Try different combinations of gearings in the formula and choose the correct gear ratio to provide the performance you want from your car.

Tips and warnings

  • Changing the gear ratio in your transmission can be difficult without replacing the entire transmission.

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