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Difficulty: MediumClutches and Transmissions

In an automotive transmission, gear ratios describe the rotational relationship between the input shaft and the output shaft. Which of the following gear ratios represents an overdrive condition where the output shaft rotates faster than the input shaft?

  1. A
    1.00:11.00:1
  2. 0.75:10.75:1Answer
  3. C
    2.45:12.45:1
  4. D
    3.80:13.80:1

Answer

0.75:10.75:1
A gear ratio of 0.75:10.75:1 indicates that the input shaft makes only 0.750.75 turns for every 1.001.00 complete turn of the output shaft. Because the output shaft rotates faster than the engine input shaft, this is an overdrive ratio, designed to lower engine cruising RPM and improve fuel economy.

Step-by-Step Solution

1
Understand the standard definition of transmission gear ratio notation
Gear ratios are expressed as (Input Shaft Revolutions) : (Output Shaft Revolutions).
Establishing the correct reference point is required to determine whether a gear multiplies torque or increases rotational speed.
2
Evaluate the condition for an overdrive gear ratio
An overdrive gear occurs whenever the numerical ratio is less than 1.00:11.00:1 (for example, 0.75:10.75:1).
When the ratio is below 1:11:1, the output shaft turns faster than the engine crankshaft, reducing engine RPM at highway speeds.

Key Concept

Transmission Overdrive Gear Ratios
Estimated Time:1m 0s
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Topics

Clutches and Transmissions

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